The only real catch here is that the newest releases aren’t available to rent. There appears to be about a one week lag before the newest release are available.
Review excerpt: “I was very impressed with the quality of the rental service on the Apple TV. Once I choose the movie I wanted to watch the download began immediately. I expected a long wait while the film downloaded and buffered. To my surprise the movie was ready to watch in exactly 49 seconds according to my trusty stopwatch.” Read the full Apple TV movie rental review here.