Obviously, the hardware is very nice - the screen is beautiful (super-high resolution and very nice colours), and everything is very snappy, quick, and very, very pretty (which means the graphics bits must be doing some hard work). The size (4.7 inches) is ideal, it is not so big as to cause thumb strain, but the screen is very 'visible', it feels a lot more usable for a lot more things than the iPhone or similar-size devices. The main camera is really good, but the flash is a bit green for close ups, the forward facing camera (for Skype etc.) has poorer colour balance and is a little fish eyed, but still better than a standard webcam.
The Android and toned-down-Sense UI work very well, so much less clunky than older versions of Android. Most of the UI is very easy to use and intuitive. I would go so far as to say it is a better UI than iOS, but maybe that is because I have a bit of a geeky mindset. All the basic utilities and OS things (app selection, home screen layout, clock, calender, and so forth) work very well, the balance of summary vs detail is nearly always just right. There is great consistency, the same gestures tend to do the same things, wherever you are, and they seem more easily discoverable than on the iPhone. Having three perma-buttons, rather than one is a win, I think, although having a menu button (like the Samsung phones) would probably be good, since all the apps have one, but often in different places. Actually the recent apps (kind of task switching) button could probably have been got rid of, I only use it rarely. The mail application could do with some work though: I want to delete an email with one click and without reading the message, and I want to see all my new mail (from multiple accounts) in one place, and I want more than one message on my lock screen.
The thing I really like (and which really differentiates it from the iPhone) is how everything is so customisable. Everything I have wanted to change so far, I have been able to (actually, except the home screen clock, if you want to keep the weather forecast with it - you can make it smaller, but are stuck with the calender-style digits, and the aforesaid mail-on-lock-screen thing). And, you can customise without ruining the look and feel, a very impressive balance has been struck!
On the downside, I guess the UI guidelines are not as strictly enforced across apps as on the iPhone, some apps should be a bit more standard.
On to the fun stuff; it was a bit of effort to get a debug link to the phone. Setting the phone up was easy, but getting it to talk to the laptop, not so much. There are a lot of options for connecting, and it is unclear which to use for development work. HTC don't make available the required drivers either, so you have to install their sync software, which includes the drivers, then sync (which tries to auto-run) conflicts with ADB, so getting ADB to work reliably was tricky, eventually I got it working, and managed to install a nightly version of Fennec (mobile Firefox), so the path now is clear (hopefully). I'm looking forward to getting a debug version of Fennec on the phone and working out how to debug Android FF, but that first requires building Android FF locally, which is no picnic (top tip, you can download the Android apk from the Try server instead). I also installed the NDK plugin for Eclipse, so all I need now is a little bit of time and I can hack up some Android apps - fun times!
So, they say smart phones live or die by their apps, and so far my experience of Android apps has been good. All the ones I've downloaded have been very polished, usable, and powerful. Just as much as the equivalent iPhone apps, if not more. Plus they all tend to be fairly customisable, just like the UI. In fact using the app on my phone is often a more attractive way of accessing a service than using the website on my laptop. If anyone has recommendations for apps I should be using, please leave a comment!
Of course the real selling point of Android over iPhone is the openness of the system vs. living in a walled garden. I can't imagine buying a phone I can't put my own programs on, it just seems ridiculous. But, so far, other than the nightly version of Firefox, all the apps I've installed have been through the Google store, so I guess the practical affects are minor.
And last, but not least, the web. Obviously the mobile web is a big thing right now, and especially for Mozilla. I found mobile web disappointing. I tried a whole bunch of browsers, and none of them were as well-adjusted for mobile as I would like, they all felt like they needed some tweaking. All had bugs too, unlike desktop where you rarely notice bugs in any modern browser. More importantly, websites need to have mobile versions. Desktop versions of websites just don't work very well on mobile, mostly the problem is screen size, but there are other things, slow internet connection and slow processor cause problems too. When websites do make mobile versions they, sometimes use redirecting, which is annoying, having to wait for another page load (even worse when they ask you first). Or the mobile site is designed for iPhones or devices with similar sized screens, so is actually too big on the One X. I'm sure it is possible to do this right, but very few websites manage it, and using apps is nearly always a better experience. I hope that this changes, but as it requires some intelligence and effort, I'm not hopeful.