Although gameplay is almost constantly shifting styles, Phineas and Ferb: Across the 2nd Dimension
manages to provide a solid experience throughout. Controls are, for the most part, solid and incredibly easy to learn. In-game actions may be different, but the control mechanisms are constant and crisp.
You're joined in each level by another character. Though the addition of a second human player decreases difficulty even further, two players are always better than one. If you can't find someone to play with, the A.I. does a good job of keeping up. Each character has their own unique abilities, so there are differences, though it isn't likely you'll notice them while playing. For example, Phineas regenerates health over time, yet there are health packs everywhere, negating the bonus to playing with the character.
Across the 2nd Dimension absolutely nails the show's gadget concept. You'll come across several gadgets in your dimension-jumping adventure, each with a set of upgrades. New parts are scattered in out-of-the-way portions of each level (making them one of the few reasons to replay completed levels) and can really add some punch to your weapons. Not that you really need them, but who doesn't like upgrades?
Phineas and Ferb: Across the 2nd Dimension is an okay pick-up, though only for parents with really young kids who are fans of the show. Although the series has its older audience, the lack of difficulty is a deal-breaker.