Just finished watching our first movie rental using the Apple TV: the HD version of Michael Clayton.
The picture was pretty good - the stubble on George Clooney's face seemed particularly sharp - but without the SD version to do a direct comparison with, it was difficult to tell whether it was sufficiently better to justify the additional $1 premium. That said, the HD version did come with Dolby Digital 5.1 surround surround too.
I set it downloading when I went to bed at midnight last night, and by 8am this morning, it was done. That was much faster than I was expecting, although the movie was only 2 hours long.
At $5 for a new release, it's a little pricey compared to the $2.50 you'd pay to rent a DVD from Leisure Time, but the convenience, and the absence of any late fees (which we inevitably end up paying when we rent a movie at the weekend) just about compensate, I think.
As for the movie itself, we enjoyed it, but thought the plot was entirely predictable.