Well after trying it, I have learned a few things.
One is that it's exactly like the movie rental service. You get a game in the mail, and can keep it for as long as needed.
Second is that I'm going to have to limit it to one game at a time for my own consumption. I just don't have the time to play two games to completion at one time lol. That's more of a downfall for myself than the service as I can't commit the time to video games that I could before kids. :) It's not a bad thing though.
Third is that the game selection isn't that great right now. All new releases are on a two month wait before they are available on the online service, which sort of sucks, but once again, since I don't play a lot of games, I have plenty of older titles that I wanted to try out to catch up on and by the time the newer games come out, I won't have to worry because I can wait.
So overall after the first week of the service, I found it a great value if you are already a Blockbuster Online user. It's both cheaper than Gamefly, and Netflix, if you use them in conjunction, so that's a bonus.