Well I think PS+ has been good value for money.
I have the minis, the ps one games and the free games in Sam & Max, Zen pinball (a bit crap) and Wipeout HD (Excellent) and then LBP all for £36 (used PSN cards).
I would have none of these if I hadn't subscribed as I wouldn't buy from the store normally.
Then there is the other content (themes etc) and automatic downloads - this is worth it alone as the number of times I have gone to play a game only to find it needs to update before i play , thus not being able to play when I wanted.
I am a happy bunny with only 1/3rd of my subscription used and like the fact that I am getting content I wouldn't normally buy as it gives me a chance to sample different things.
I guess if you are a regular buyer of games (I am not) then its a bit hit and miss, in case you already have them games being offered.