Not worth buying.

User Rating: 4.5 | Ben 10: Alien Force PS2
It doesn't really matter whether you're a fan of the TV series or you just want to kill a couple of hours beating up bad aliens in Ben 10: Alien Force The Game because you won't enjoy anything the game has to offer. The story is quite terrible and doesn't relate to the series (aside from the main characters that are present in the game), the movement of your character is quite confusing, making it easy to miss jumping on certain objects (which will result most of the time in falling in to the water and dieing) and the gameplay is probably the most repetitive and annoying one I've seen so far. Cutscenes are poorly done with lots of blurry textures and darn right annoying voice acting.
You can play as Ben, his cousin Gwen and the local bad-boy Kevin. The game has co-op which is good for when you get killed because you don't have to restart the mission but the two players (2 x Ben, 2 x Gwen or 2 x Kevin) have no differences to tell them apart making the gameplay even more confusing then it already is.
The only available aliens that Ben can transform in to are Swampfire, Big Chill, Spidermonkey, Jet Ray and Humungousaur which is pretty disappointing but the combos they all have aren't that bad so I guess that kinda makes things a bit better and adds a bit of enjoyment.
Overall the game really isn't worth a buy ... Maybe a rent if you're curious enough to try it.