As a game known for easy achievements, it's not bad, especially for a licensed title. You alternate levels between Penny (who's best at sneaking and working behind the scenes) and Bolt, who's definitely the brawler. Combat with Bolt is surprisingly well-done, with some clever touches (such as the laser eyes and the finishing moves).
Most of the levels are easy enough, though there are a few that will try the patience of even the best gamer (the train and the final missile sequence come to mind). Cleverly integrated into the gameplay are hacking minigames, which take the form of a Geometry Wars-style shooter.
The majority of the achievements are pretty easy, as are most of the secret ones. I would suggest a guide for the power-ups...most of them are in plain sight, but some of them are very well-hidden. With a good guide and some patience, 1000 is possible in one sitting. See the achievement tips on this site for a few pointers on the trickier ones (such as Sneaky Sneaky).
Best of luck. I picked this up for $10 at Best Buy for my son and ended up enjoying it myself. Not the best game, but a reasonably good distraction.