I have been using a Palm Pre (Plus) for a while. It is a great device; the form factor is excellent with a hardware keyboard and I really like WebOS. Unfortunately, the build quality is not so great so it always felt a bit cheap. Additionally, it was also always kind of on the slow side (it took much longer for it to render HTML than it took to download it, and many other operations were slower still).
HP basically killed off WebOS and there is no new hardware on that front, so that basically left me to choose which Android phone I wanted. Luckily, AT&T was gracious enough to push the release date of the HTC One X until after Samsung's Galaxy S III launch event so I could see what the big two had in store this year. Also fortunately, Samsung's offering was disappointing enough that I could jump on the HTC One X without reservations. HTC definitely won on the design front and the internals were close enough to not matter to me. Samsung's only advantage was including a microSD slot, but my on-phone storage needs are modest enough for that to not factor into my calculations at all. Also, based on my interactions with Galaxy Nexuses, the One X screen is much nicer.
Initially, I was a bit worried about getting a phone this large. Fortunately, my hands are large enough to deal with it. I never want to see a phone larger than this 4.7 inches, though. Despite its size, it actually feels lighter than my Pre did, which is pretty weird. As everyone else has noted, the screen is stunning and far crisper than any SuperAMOLED screen I've ever seen. The phone is also very fast. Coming from my Pre, it is like night and day. I'm not used to seeing such responsiveness in a mobile device.
Many people expressed concern over the battery life. My usage patterns are probably weird (I don't want notifications and I don't poke at it every few minutes), so I can get three full days on a charge. The non-removable battery doesn't bother me -- I only removed the battery in my Pre once and that was to get it to reboot after a lockup. I suppose most of the battery life comes from Ice Cream Sandwich being pretty smart and the newer manufacturing process in the newer Snapdragon chips. Battery life was my biggest concern when upgrading to a modern phone and I am pleased that such concerns were completely unfounded.
As for software, I never used any prior versions of Android. I also haven't had many interactions with stock ICS, so I can't really make any comparisons to other Android versions. I do see why some people complain that Android is a bit inconsistent and scattered compared to WebOS. I think I even agree with that assessment, but I think it is mostly because Android just provides so many more features and so much more configuration options than WebOS. Part of the reason WebOS could be so simple and consistent was just that there was less to do and fit on screen. I haven't had any trouble adapting to ICS (with HTC's Sense mods). I've read that Sense used to be a gaudy monster, but the ICS version doesn't seem to have any problems like that. Everything also seems much more polished and featureful than WebOS, which isn't surprising considering the disparity in resources applied to the two systems.
Overall, I am quite happy with this phone. Good job HTC.