Yesterday, T-Mobile and Google launched the G1, previously known as the HTC Dream, the first Android-compatible mobile phone. Comparisons with the 3G iPhone were immediate and obvious; the G1 is about the same height and width as the iPhone, but it's about twice as thick. It will sell for a little less, $179 vs. $199 for the 3G iPhone, and like the iPhone is available only on a two-year plan. The G1 has several more buttons than the iPhone, the most important of which are part of a slide-out QWERTY keyboard, which, of course, the iPhone doesn't have.
As you'd expect, the G1 is very Google-centric; you have to have a Gmail account in order to use the phone, and all the other Google services are front-and-center. It has a lot of location-centric, GPS-based features--even more than the iPhone, and unlike the iPhone, turn-by-turn navigation applications will be possible. And, developers can add applications to the G1 without review or approval by Google or T-Mobile, so it's a far more open platform than the iPhone.
There is a "but", however, and as this video from Engadget shows, in this case the "but" is that the iPhone is just much better integrated, and operates much more smoothly, than the G1. However, that's to be expected, since Google's Android is designed to operate on a wide variety of devices, some of which Google will have very little control over, while the iPhone and the version of OS X that the iPhone runs are engineered together. Android is a "generalist" system, while the iPhone is very tightly integrated.
I'll withhold my final verdict until I can play with the G1 myself, but from everything I've seen, it's not as refined as either the iPhone or the most recent BlackBerry models, but nevertheless, it's a very good first effort.