To put it very simply, whether or not you like Secret Agent Clank is going to depend on how much time you're willing to put into mastering the PSP.
There's really no such thing as a bad Ratchet & Clank game; the worst thing you can say about something like, say, Deadlocked is that it didn't have the same irreverent sense of humor as the others. As action-adventure games go, they're always solid bets. I just don't have it in me to be hard on a game that lets me loose with this much firepower.
Secret Agent Clank isn't an exception to that. It's often funny and contains a lot of varied gameplay, from giant monster fights in the middle of major cities to classic Ratchet shootouts to the relatively new introduction of stealth-based action. It's a testament to the flexibility of the game's design that the latter component doesn't shoot the entire game in the foot; it's implemented into the same engine without feeling like it was shoehorned in.
There's a "but" coming up here, naturally, and that "but" is simple: this is a PSP game.
Secret Agent Clank begins when Ratchet is arrested for the theft of a massive diamond from a museum. What's worse is that he's caught on camera confessing to the crime, and Clank watches the police drag him away. With no idea why Ratchet would go from galactic hero to cat burglar practically overnight, Clank is forced to draw upon his training and experience as an intergalactic secret agent. I'm sure he has had this the entire time.
The game is essentially split up into several parts. As Clank, you get to investigate various locations as you track down the mystery of why Ratchet went bad, using stealth and gadgetry to your advantage. As Ratchet, you play combat-based minigames in the traditional Ratchet & Clank style, which is to say, you shoot everything that moves and a fair amount of things that don't. As Clank's squad of Gadgebots, you'll solve a variety of puzzles using various robotic abilities. Finally, as Captain Qwark, you'll wield the twin powers of superheroic might and being a tremendous idiot as he dictates his autobiography.
If you've ever played a Ratchet & Clank game on the PS2, you're in good shape for this. The series is easy to pick up and fun for pretty much anyone, as it's essentially a shooter dressed up like a platformer. The animal mascot and bizarre-looking robots are a sort of camouflage for what can be a surprisingly intense action game, featuring a wide variety of gadgets and weaponry to help vary the experience. The series's inherent simplicity has always been its strength; it takes a pretty entry-level kind of gameplay and does it very well. Insomniac may not have been the developers on this one, but you wouldn't be able to guess that just by looking at it.
The problem, as with so many games, is that this is on the PSP. The traditional Ratchet format involves fast-paced, rapidly-evolving situations in a three-dimensional space. If it's a gunfight, that means you need to circle-strafe; if it's a stealth-based section like Clank's, that means you need constant and exact control of the camera in order to keep track of the people and/or objects you're sneaking past.
On the PS2 and PS3, this is achieved with the right analog stick. On the PSP, which unaccountably lacks such a thing, many games have been routinely torpedoed by the lack thereof. Secret Agent Clank is better off than most; it uses the D-pad for strafing and the shoulder buttons to reset the camera, which means you aren't crippled by the lack after you undergo a certain period of adjustment. It copes with the problem more elegantly than some other games would.
The presence of that period of readjustment, though, is my biggest complaint about Secret Agent Clank. Like a lot of developers, High Impact has made a top-shelf PS2 game... on the PSP. The opening level, as a result, is a much bigger pain in the ass than it has any right to be, and subsequent stealth-based levels are often pure exercises in frustration as the camera refuses to cooperate as smoothly as it ought to.
There are obviously quite a few people who've managed to overcome this problem in the PSP, to judge by the sales figures and critical acclaim you see for games like the Syphon Filter series. I have a hard time moving past it, as even at its best, I find myself wishing a given game was on the PS2 instead. If you're able to deal with the PSP's control limitations, then I can give Secret Agent Clank a completely unqualified recommendation. If you're like me, though... well, you probably don't have a PSP anymore.