Underrated !
As an independant game designer : I also bought Teen Titans, which is rated 6.6 on Gamespot. But slow framerate and unresponsive controls make TT a game to avoid.
Ben 10, however, have very intuitive and responding controls, very nice use of the cell shading technology, some well made puzzles alongside and impressive boss battles.
There are only 5 aliens. I can understand it's a major letdown for all the Ben 10 fans. But the 5 aliens included in the game are incredibly well rendered. All your favorite villains are in there (except my son's favorite, CharmCaster) and the game seems like a long episode.
For a PS2 game, I was quite impressed with what I saw. Environnements are well rendered. Sure, it is a bit repetitive, but the game has enough to please Ben 10 fans.