Here's the problem with PS+ as an alternative to buying games: I have too much f--king stuff to play.
I'm still stupid early in Saint's Row 3, have just about gotten to grips with XCOM again, and just about finished the tutorial in Kingdoms of Amalur; meanwhile I've not even gotten around to downloading Battlefield 3 (I only remembered that it's there after seeing it on the store homepage) and didn't give Demon's Souls a proper shake because I knew I'd have more fun with other stuff I've yet to finish. Not to mention my rapid abandonment of Hitman, Arkham City and Vanquish (although I'd finished the latter two on 360 previously). And then there's the arcadey stuff like Joe Danger and LBP Karting that I've still not gotten around to.
And now, in August, they're giving me another sandbox game in Mafia II (I platinumed Sleeping Dogs and am having enough fun with Saint's Row to see it through at least to the end of the story), Spec Ops: The Line (which I'm partway through on PC but will probably restart on PS3 anyway), and Need For Speed: Most Wanted, which I was quite intrigued by, pre-release.
I've only actually managed to finish Catherine and Okami so far, both of which I'd already beaten on other platforms.
I should definitely get a bigger hard drive. Having to delete stuff to make room for other games has really killed my ability to drop-in to stuff that I might otherwise have stuck with.