My only major complaint is that it's a little too big. The display is 5.2 inches, i think i would be much happier with a display of 4.7 inches. Anything between 4.3 and 4.8 would probably be ok.
Other than that, it's great.
A little history of my previous smartphones; this is my 4th one. The first smartphone i got was a Nexus One (G1) with a physical keyboard. I loved it. The second one i got was a high-spec smartphone without a physical keyboard (Motorola Atrix). I hated it; typing on the soft keyboard led to too many mistakes. The third one i got was a low-spec smartphone with a physical keyword (LG Optimus F3Q). I loved it. So, this time around i figured that my main requirement was to find a phone with a physical keyboard. The Nexus 5x does NOT have a physical keyboard, so i initially ignored it. However, after trying it out in stores, i realized that i could indeed type accurately on this keyboard. In fact, i can now type more accurately on soft keyboards than on portrait-mode physical keyboards such as on the Blackberry Priv (my previous phone, the LG Optimus F3Q had a 5-row landscape keyboard; it was a slider; my typing accuracy on that was comparable to the Nexus 5x's soft keyboard). So, i guess the problem with the Atrix must have been that phones of that generation were still so slow that typing on the soft keyboard was laggy; but now that phones are faster, the soft keyboard is good enough for me. In fact, i even prefer it, because you don't have to press the keys so hard. So, if you are like me and have been demanding only phones with physical keyboards, give the soft keyboard another try, you may find that it's finally good enough this generation.
Software: Android 6 is wonderful.
CPU: The CPU is fast enough. I've never noticed any lag.
(rear-facing) Camera: The camera is good. Then sensor size is 1/2.3" with 12.3MP (that's 1.55µm pixels) (note that sensor size is as important as megapixels, and too many megapixels can actually be bad if the sensor is not large enough to support them, eg if the micros per pixel is too low). Taking a test picture of clouds at twilight (after the sun set), I was surprised by how sharp it was given that level of light and given that it is a phone. My Sony RX100 still takes better pictures (of course, given its much larger sensor size), but the Nexus 5x is good enough that it can be my go-everywhere camera (in sharp contrast to my previous low-spec phone, the LG Optimus F3Q, whose camera was so bad that i would almost never use it even if i saw something that i wanted to take a picture of and had no other camera). The camera is fast enough (in sharp contrast to my LG Optimus F3Q, which had a long lag). I like the built-in camera app for the most part (and i LOVE phonespheres), but i would prefer if it had an option to manually set the ISO and shutter speed. I would prefer if the camera (or at least the camera app, as in the Nexus 6) had image stabilization.
front-facing camera: haven't really used it yet. Seems to be pretty good.
RAM: I was worried about the 2GB RAM, since my previous phone had 1GB RAM and had a lot of problems with that, but so far it seems to be handling it well. Large games do swap in and out of memory each time you put them in the foreground, but other than that i have not yet noticed any memory-related effects (although maybe they're happening and i'm not noticing them). When you haven't recently played a large game, the free memory seems to be around 500M. The Samsung Galaxy has 3GB, but on the AT&T store demo S6, memory usage also seemed to be around 500M, so i guess that maybe the Samsung bloatware eats up the extra GB anyway. Still, i would prefer if the Nexus 5x had 3GB RAM.
I would like if the auto-rotation between portrait and landscape occurred faster (ie with less latency).
Storage: I got the 32GB version. I don't watch many movies or listen to much music on my phone so i probably won't need more than that. I do sort of miss the microSD card, because that made it easier to move files between my phone and my computer, but it's not a huge deal.
Battery: I would greatly prefer a removable battery. Since battery life decreases as devices age, and since with each of my previous phones i've ended up buying a new battery (which costs about $15) about halfway through their lifespan, i expect that with the Nexus 5x i will have to buy a new phone at that point instead; this probably almost HALVES the expected lifespan of my device, forcing me to shell out hundreds of dollars for a new device years sooner than i otherwise would have (greatly decreasing the worth of the Nexus 5x to me). Also, with other phones i usually carry around a spare fully-charged battery in my pocket on trips, allowing me to not stress about using up the primary battery and being without a phone while on a trip. The workaround for non-removable batteries are external batteries (power banks/power packs/charger cases/boost cases), but these are bulkier to carry around than just an extra battery, so you might take one in your suitcase, but not in your pocket. My Nexus 5x has just enough battery to last a day; since battery life decreases as devices age, since the Nexus 5x has just enough battery to last a day, this means that my Nexus 5x's battery will NOT last a day when it gets older. Another problem with a non-removable battery is that if the OS ever manages to hang so hard that the software-mediated powerdown doesn't work, there will be nothing i can do aside from waiting hours for the device to use up its battery. This has happenend to me on previous Android devices (and it happens to me on my laptop), so i'm a little worried.
Battery size (battery life) is decent. I can drive for about an hour while using Google Maps turn-by-turn navigation the whole time (which consumes about 30%), spend all day somewhere, then drive back for an hour again with Google Maps navigation on the whole time, and the battery lasts the whole day (although it is almost out by the time i go to sleep). This is in sharp contrast to my previous phone, where doing this would require that i fiddle with the phone a lot, turning off GPS when i'm not using it, going into airplane mode when possible, etc in order to save enough battery for the trip home. If you watch movies or play games this quickly uses up the battery, but i don't. I would still prefer if battery life was longer; battery life becomes particularly important when the battery is non-removable. I would prefer for the phone to be thicker and heavier in exchange for longer battery life. Ideally, for a non-removable battery, the battery would last through 2 days of my normal use when the device is new; this would probably translate to a battery life >1 day of normal use for the entire time that i would otherwise want to use the device. If the battery were removable then there would not be such a need for long battery life.
Charging speed: fast
Fingerprint reader: i find this really useful, and i like the location on the rear of the phone, rather than the front bottom. It works most of the time (it fails to recognize my fingerprint about 1 in 10 times, but if i then place my finger more carefully solidly on the sensor, it almost always gets it; i've only had to fallback to the password a few times (don't forget your password though; if your fingerprint fails to recognize more than a few times it locks out the fingerprint reader and demands a password)). I would prefer if you could register more than 5 fingers; i would like maybe 12 (12 would let me register all of my own fingers that i use, plus a few dupes to improve recognition, plus a few friends and family's).
Display resolution and colors and brightness: Resolution: 1080p (1920x1080). Imo this is the perfect resolution. On a phone, i end up zooming in a lot anyways to see text; if you make the resolution any higher than this, you needlessly consume GPU/CPU power and battery life. Colors: i love the colors. Apparently, because this is an IPS screen and not an AMOLED, the colors are less garish and more accurate, which i really like. Brightness: it is bright enough.
Shape and materials: The back is plastic. I prefer this because it's light and not too slippery (it could stand to be even less slippery, though) and durable, but some reviewers seem to dislike plastic, apparently just because they feel it isn't fancy enough. There is a camera bump on the back, right above the fingerprint reader. I don't mind this, but it does mean that your finger often gets on the camera lens when you are trying to reach the fingerprint reader; if you really want high-quality photos then you'll want to keep your camera free of fingerprints in which case this would annoy you (i don't care enough about photo quality to mind). The phone is rather thin and lightweight for its largish size. Apparently Gorilla Glass 3 is used, which is less durable than Gorilla Glass 4. However, comparing this phone to the Samsung S6, which has Gorilla Glass 4, i found it easier to drag stuff on the screen with my finger (less friction) on the Nexus 5x than on the Samsung S6, so the Nexus did something right (not sure if this is due to the Gorilla Glass 3, or some coating on top of it, or something else).
USB-C connector: Note that the USB-C connector is not compatible with existing USB cords unless you buy an adaptor, but that's fine, because my guess is that everything is going to be switching to USB-C, so you may as well make the transition sooner rather than later. The USB-C connector snaps in, which i prefer to USB-B; another nice thing about USB-C is that it goes in either way (upside down or not). Note that USB-C video out is NOT supported. The phone comes with a USB-C cable and a charger.
If you are shopping for phones, i found the website http://www.gsmarena.com/ to be the best one for the purpose of just looking up the raw specs for each candidate, and also for doing an initial search for candidates according to spec criteria (including what Android version is available).
Summary of my complaints:
Decent things:
Great things:
overall impression: i love it. It would be almost perfect if it were a little smaller and had a removable battery.
Settings: wifi: setup your local wifi networks more: emergency broadcasts: all off (NOTE: i think the presidential broadcasts may still be on; i think you have to root your phone to turn that off (!)) networks more: android beam: off
location: mode: high accuracy Google Location History: off (i thought this was cool, but [1]