I've had this since the release date because I felt like buying a new gadget and my old phone was kinda crummy.
The interface is nice, although the live wallpapers suck up a lot of battery so I turned them off right away. The way the homescreens are arranged are intuitive and makes certain things easy, like checking the weather, and managing the phone's major functions (wifi, bluetooth, GPS, screen dimming and syncing can be controlled with a single button press). Setting it up to sync with Facebook / Gmail is easy, and the notification system is useful. Call quality isn't too bad, the standard ringer sounds are nice, and the vibrate mode can be felt even when the phone's inside the soft material cover that it comes with.
For a while the browser system was pretty bad, as there was no multitouch and I was using a third party one (Dolphin). It was annoying because whenever I would open a new page, it would always open in Dolphin even if I was using the native browser. The multitouch patch has fixed that and the Google browser works pretty well now. Though I'm still holding out for Opera Mobile and it's flash support.
Porting numbers was a pain in the ass, partly because this phone seems geared towards people who use Google as their rolodex. Transferring numbers over from my SIM card took almost a day and was a pretty confusing process. The lack of any kind of manual or support didn't help matters.
The phone layout is also a little odd. The trackball glows when there's an unread notification, but other than that I don't use it too often. There's a nice and large touchscreen, but no dedicated buttons to make or end a call (every other phone I've had had these, and it's what I miss the most). You can get around it by making shortcuts on a homescreen that call someone directly, but it takes some getting used to. The battery can quickly die with lots of features turned on, but by careful monitoring of the running applications (KillApp is handy for this), you can easily squeeze out 2-3 days worth of idling and 5-6 hours of talking/web surfing.
By far my favorite thing about this phone is the voice integration, though. The way it's integrated into Google Maps makes the phone into an excellent GPS, and it was a little freaky how well (and how easily) you can find a specific address. It's also extremely helpful for web browsing. and is an absolute win for Android.
My least favorite thing is the way media is handled. Although I finally get to dump iTunes (which has only gotten worse with every incarnation IMO), there's just no good media player app for Android yet. The native one is passable, but there's no decent 3rd party app that integrates audio, video and pictures well (Nemoplayer is one, but you can't create playlists and it's got very limited features).
Overall the hardware makes for a pretty sweet handset (that should last a couple of years at least), but the phone rises and falls with the OS. What I like is that software updates are continuous and will (likely) be available over the air. Android shows a lot of promise right now, and they've gotten a lot of things right with 2.1. With the addition of Flash support, better media handling, the release of Opera Mobile, and continuing system updates, this phone has yet to reach it's full potential. I only hope that T-mobile will be rolling out their new 3.5G/4G networks soon* so that I will be able to stream World Cup games to my phone at work.
* Yeah, I'm thinking that Verizon is probably the best carrier for this, as they seem to have the fastest implementation roadmap for their LTE network.