There are a couple things that bother me about the service:
1. It's annoying to have to go in and claim the free games each month.
2. It's annoying that I have to manually cancel my membership (i.e., they don't make it easy to buy a month service without having to go in and cancel it for future months)
With that out of the way, their service is pretty good, and I get to play a lot of games that I wouldn't have played otherwise. Because of PS Plus, I re-discovered Heavy Rain, which I hadn't finished when I first played it. I enjoyed it much more this time around, and even went ahead and got Beyond Two Souls and finished that one as well, all leading up to Detroit.
Most months the games don't interest me, but on occasion something great comes along.