Since I've got myself a T-Mobile G1, I've had several people ask me if they too should get one. And the answer, as it often is, is: It depends.
If you want a phone with good battery life, this is definitely not the phone for you. If you have GPS and Wifi and 3G and all the bells and whistles turned on and you're using the phone constantly, you'll probably only get four or five hours of use out of it. I turn GPS and Wifi off and turn the screen brightness down (because even when it's turned right down it's pretty bright), and don't use it non-stop, just a fair bit of twittering and the odd bit of internet browsing and some texts and phone calls. And usually it's down to around 40% battery by the end of the day. Of course, it depends completely on how you use it. I usually plug the phone in at night so that it's fully charged in the morning. Last night I forgot to switch the charger on, and when I checked in the morning it had used less than 5% of the battery, so it's standby use is pretty good.
If you want a phone which was take videos, this isn't (yet) the phone for you. There are rumours of software upgrades on the way which will add this capability, but it isn't there yet.
If you want a phone with an on-screen keyboard, this also isn't (yet) the phone for you. It's apparently on the way though. An I don't understand why anyone would rather use an on-screen keyboard when you can use the G1's fantastic real keyboard. I didn't think I'd like using the keyboard much, but it works brilliantly, at least for my hands. The 'chin' on the phone gets only very slightly in the way. And my phone is a black one - I've heard the letters are hard to see on the white and bronze versions.
If you want a phone which doesn't force you to have a Google account, this isn't the phone for you. The first thing you have to do when you set up your phone is either enter your Google account details, or sign up for a new account. The phone is very tied into the Google way of doing things. This isn't that much of a problem for me - I'm not Googlephobic. Nor am I a Googlephile. Most of my email still goes through my own server, and there is software for the G1 which lets me read that email just fine. And I was starting to use Google calendar just before I got the G1, so it's quite nice that it is now synched up with the phone. Some people will find the Googleness of the phone to be a hindrance, but I find it works really smoothly, and is around three million times better than my previous Nokia when it comes to synchronization.
If you want something which is thin and stylish, this probably isn't the phone for you. It isn't huge, but it's certainly no iphone. It's not ugly either - personally I think it's really well designed and good looking. Again, it's up to the beholder and their eye.
If you want a phone with an incredibly flexible operating system, this is the phone for you. The Android OS is very well designed, and the way it operates is mostly very intuitive. And unlike Nokia phones, where it seems that once you have the phone, you won't get any new features until you buy a new phone, the T-Mobile say that the G1 will be actively updated with new versions of software.
If you want to write software for your phone (in Java), this is the phone for you. Writing Android software is very easy, and Linux is a first class operating system when it comes to their development tools. Some parts of the SDK aren't quite finished, but that's because Google hadn't got them 100% stable before releasing the phone, and thought it'd be better to release a stable incomplete SDK that a buggy complete SDK, and I think that's a good way for things to be.
If you want a phone which is exceptionally useful, this is the phone for you. There is already a huge amount of software available for it, and although there's a fair amount of rubbish, there's also loads of very useful stuff, and I'm finding the phone more and more indispensable.
If you want to be in on the ground floor with the mobile operating system of the future, this is the phone for you. I genuinely believe that Android is the way to go with mobile phones, and have no regrets about getting this phone. And I purposefully got a twelve month contract rather than anything longer, because I'm pretty sure that by this time next year there'll be Android phones out there which are ten times better than this one!
The phone's main negatives for me: poor battery life; not enough internal memory; sometimes a little sluggish going back to the home screen (I think it's probably garbage collecting); a few minor niggles with some of the core software (but at least I can branch the code and write a better version if I want to).
The phone's main positives for me: bright, high contrast, high definition screen; real keyboard; sensitive (capacitive) touch screen; well designed operating system; mostly well designed software; easy syncing with Google; infinite possibilities; easy to use; fun to write software for it.