I've rented one HD movie: Batman Begins. I thought it looked great, but I don't have Blu-ray for comparison. It definitely looked better than DVD. It only took about two minutes before it said it was ready to be viewed (it showed 2% done at the time). It took about 100 minutes for the complete download (I watched it later). The download time is going to depend heavily on the speed of your internet connection. I've got Roadrunner cable, and it's pretty fast (don't know how fast, though).
If you want to do more research, there's a forum over at MacRumors dedicated to AppleTV:
Apple TV - Mac Forums