This is a great beginning for PC gamers. Please stop comparing this to 'renting a game for a week' because that option is sadly limited to consoles (unless someone knows of an alternative I don't know about, please inform me if there is one currently), not PC games. Obviously, there are a lot of ways to rent games for consoles, but there aren't many ways to rent PC games. Not everyone who is a gamer owns a dedicated console, and if they add more titles and the ability to extend the rental period at 5 hour increments, this may be a great way to try out or play games.
I can picture 3 great uses for this service:
1) Try out a game you think you might buy to see if you like it (especially multiplayer games, which generally aren't covered well in reviews or Let's Play type videos)
2) See if your gaming rig can actually play a particular PC game
3) Pay only $5 to play a particularly short game so you don't feel obligated to pay $40-50 for a game less than 5 hours (and if they add the ability to extend a rental, you could play a <10 hour game for still less than full price)
For any of those 3 purposes, this is a great way to get PC gaming rentals started. It still sucks for PC gamers who don't have internet access, though. There is something to be said for those demo CDs that used to be in gaming magazines...