The good news is, Ratchet & Clank: Size Matters is a very faithful entry in the Ratchet & Clank Series. The creators didn't try to do anything too outlandish, tamper with the game to 'be different', or strip away any of the PS2 release in order to fit it into the PSP (this may be due to some claims I've read that the PS2 version was a port of this PSP version).
That said, there are severe limitations to this title that all seem to center around having to use a PSP to play it. I love my PSP, don't get me wrong: but this platformer is not easy to wrangle on it. I seem to be constantly suffering from camera problems: slow panning when I call for it, and quick snapping away from the scene when I'm just moving around means that a lot of times I end up having to run away dodging so I can put distance between myself and my opponents and buy some time to get them back in my sights. There is a "strafing mechanic", but heaven help you if you start off at the wrong angle. There is a "snap the camera to center" mechanic, but fumbling for it during combat is awkward.
There is also some difficulty with button-mapping. Jumping, high-jumping with mechanical assistance, and long-jumping use the same button mappings. Trying to time the act of running, tapping the shoulder buttons, and simultaneously hitting the 'X' button can be really difficult--and performing the wrong type of jump can be either frustrating or fatal.
For those reasons, I don't recommend that someone new to Ratchet and Clank start here. Try some of the newer entries in the series to get used to the gameplay, then get ready to have to make some accommodations to do on the PSP what you're used to doing on the PlayStation.
If you're new to Ratchet & Clank, the concept is a simple one: Ratchet is (he thinks!) the last of his kind. He is a whiz with gadgets and mechanics, and from day one of the series he quickly fell into the role of 'Hero', a celebrity status that he seems to enjoy. His sidekick is a clever little robot named Clank. Clank not only helps him figure things out, he serves as some of Ratchet's gadgets, transforming into jetpacks, one-man helicopters, and more. Clank also gets some play-time of his own, too: there are times when you get to navigate him around a smaller-scale world, often enlisting the help of his fellow robots. They live in a sci-fi world where there are plenty of planets to explore and all kinds of interesting life forms, most of whom want to kill you (but a few would settle for a good 3-lap race or two). Dialogue varies from funny to downright hilarious, and there is enough variety in your objectives to keep things from getting boring.
The beauty of the Ratchet & Clank series is that it marries both sides of platform heaven: Ratchet gives you the jumping-action-shooting, while Clank lets you play strategy-maze-puzzle styled areas. As a whole the series is quite fun, and each installment seems to come up with more gadgets and fun things to do than the entire run of Warner Brothers' Roadrunner/Coyote cartoons.
That said, if you're a fan of Ratchet and Clank and you know your way around, you will be pleased to see that the graphics haven't been compromised. Colors are rich and bright and sounds ring out crystal clear. You'll probably get used to the way things work on the PSP, but be prepared for some frustrating reloads in order to get where you're going.