PlayStation Plus is a special paid service that Sony provide to its PlayStation platform. The service come in the way of discounts at selected titles, free themes for your devices, avatars for PSN, some free games(full PS3 titles, PS Minis, PSone classics), early access to betas, demos and some full titles and some other services.
After using the service for some time know, I can say that I will renew it. That US$50 I paid for the year subscription have already paid itself, in the manner of discounted titles and all the free games I have already played, some of them really great games, not just some crap games who failed to sell when launched.
I don't care much about the early access, usually one week, but the free games and discounts is what pays the service to me. The fact that I can save money on long term is nice. Yes, you lose access to the free games when the subscription expires, but you have plenty of time to play the game as much you want (unless you got the game in the last day of subscription), so it doesn't bother me.
Game trials are cool too. You have the chance to play a full game for an hour, and decide to pay for it later, which is great to really feel how a game is, specially when it have no demos or the demo doesn't make justice to the game. All that just add to the service.
Another great features are the ability to cloud save your game saves so you can download them in other consoles you have an account in, which is good to have back up files in case your console goes kaput. And the ability to the PS3 to auto-update firmware and game patches automatically is the service that I would miss more. The fact I can get home from work and don't have to wait to play is awesome. It is one of those features you hoped the system had from start, but considering I get free games with it, I am pretty cool with it.
So, I believe the PS Plus service is completely worth the asked price, and you probably would receive your money back just with all the free games. All else is really a plus in this service. If you ask me, every Playstation 3 need to have this service (I don't know how the Vita works with it, though).
I mean, those are the games you get this month: