Yeah, I thought it was ok. I rented it when I first got a Vita and definitely got a few hours of fun out of it. It seemed like there were a lot of levels and it might have got a bit tedious eventually but I quite liked what I played. It's no epic but if you like shooters you might have some fun.
I'm having a lot of fun with it. I bought the Vita PS Plus bundle that came with a Vita, a year of PS Plus, and a download code for Unit 13. It shows how a third person shooter can work well on the Vita. Sure, it is kind of generic, but still a lot of fun for being free!
@tajasaurus: Download it and cancel it. It will tie it to your account and let you download any time (as long as you are still a PS Plus member). This is what I do for pretty much every free game they put out.