If you're just using PS+ for vita, I don think its quite so worth it, but depends on the games you have for vita. All the games for vita from PS+ are all games you should have got for vita anyway tbh, at least they've all been games I've had for vita.
Only real great thing I've had from PS+ for vita so far was thomas was alone, but I do love having the likes of Uncharted/Virtues Last Reward/Rayman Origins (This month), all on my account to use on vita forever (As long as I have PS+ though, ha ha). I'm hoping resistance comes to PS+ for vita eventually, as I dont think from reviews I will risk buying it, though I certainly wouldnt mind trying it for free with PS+
For PS3 though, PS+ is the best thing ever, at first I thought of it as a kinda rental service, but its been great for getting me into games I never would have bought or even heard about, so I will very much want to renew it next year. Much better than Xbox Live, essentially forcing you to buy it to go online, while PS+ is totally optional and lets you, as a smart consumer, decide whether you want it or not.