Last night I rented Pirates of the Caribbean 3 from the iTunes Store. $3.99 and about 30 minutes later, I was watching a good movie I hadn't seen before, probably wouldn't be able to sit through again, and was done with it. It worked.
The positives of the movie rental service is that it it's relatively cheap, pretty fast, and you can watch it whenever you want as long as you get to it within 30 days of the download. When I subscribed to netflix, I was amazingly happy with the home delivery service. However, since the instant download feature was not compatible with the Mac OS, I was unable to take advantage of downloading a movie quickly to my computer.
The negatives are that, yes, you have those thirty days to decide when you want to watch the movie, but once you hit that play button, you have to watch it within 24 hours before it becomes disabled. And forget watching the movie with people. At least in my case, I find it a bit uncomfortable to try and squeeze people around my 13-inch Macbook. Also, there aren't many movies to choose from yet, and many of the ones are available aren't available for purchase. What happens if you like a movie and want to purchase the movie for keeps? Hopefully as the service grows, there will be a bigger selection and the purchase availability will be better as well.
So, all in all, it's a good service if you want to watch a movie NOWNOWNOW and alone. And just one time.