The humor is spot on, but the gameplay takes a step back from Size Matters.
The Bad: Rhythm games are...interesting, Game has no consistent tone.
The Ugly: All of Ratchet's levels, a sense of disappointment from those who've played Size Matters.
One of my earlier PSP games was Secret Agent Clank, after which I got Size Matters. In retrospect, this was the best order to go, because Secret Agent Clank is a letdown compared to the previous game by High Impact.
The games humor is the number one reason to play the game. Though the game is low on plot, the quotes Clank tosses out are faithful to the source material of proudly cheesy 60's spy flicks, while Quark's tales are insane and, quite often, hilarious.
The only real issue with the story is it has little in the way of tying all these characters plots together. It feels more like a jumble than a single plot line. The variety does prevent you from becoming completely bored, but still could have seemed more interconnecting.
The pros and cons of variety also extends to gameplay. Some forms of play are great (Clank's stealth levels, Quarks levels) While others are monotonous and boring. (Ratchet's prison levels, the long and off beat rhythm game.) The weapons are also lackluster for clank, who has less in the way of run and gun that gadgets to toss at opponents. They're cool, but not as functional as, say, the rocket launcher from the last game.
In conclusion, Secret Agent Clank is a little like a comedy film: throwing everything they've got at the wall and hoping that something sticks. In the end, less comes off than the franchises last PSP installment, but still has some substance in it.