PS+ is incredible. I have two PS3s (Basement and living room) and one hard drive is over 90% full and the other over 50%, mostly due to free games. The nice thing, like others have mentioned, that once the game goes free, as long as you "Purchase" the game (for $0.00), you don't have to actually download it right away. This means if you're short on space, you can grab the game while it's free and download it later, even after it has been cycled out of PS+.
The other thing is the games they offer for free or discounts are actually usually pretty damn good. Granted, they're not going to have a brand new retail release free on PS+, but they're also not offering PS3 launch games and acting like they're doing us a favor. Take a look at the Instant Game Collection history:
http://en.wikipedia.org/wiki/Instant_Game_Collection
EDIT: I just noticed that the link I provided doesn't say which games were cycled out of PS+ and when. Despite that, know that any game that gets a PS+ freebie or deal is usually there for a month or more.
