The Ratchet & Clank series is consistently excellent, yet oddly underexposed. It hasn't quite attained the level of ridiculous public acclaim that other, similar games have; while Sony is fully aware that Ratchet is a flagship series, it doesn't seem to get the same level of multimedia blitz that its other major franchises do. It just shows up at a remarkably frequent clip (there's been at least one Ratchet game every year since 2002) and does its entertaining job, and somehow doesn't manage to get animated adaptations, tie-in novels, or breakfast cereals. It's just dependably, somehow unremarkably good.
The Ratchet & Clank Future games are unique in the series so far, in that they have a continuing story, joined together by the PS3 Network game Quest for Booty. Tools of Destruction ended with the abduction of Clank by the Zoni, a childlike yet omnipotent race of aliens working on behalf of Ratchet's old enemy Doctor Nefarious.
Clank promptly escapes from Nefarious, but then discovers he was constructed to serve as the new caretaker of the most complex machine in the universe, the Great Clock. His new abilities and responsibilities involve the safeguarding and manipulation of time itself, which is part of the reason why Nefarious abducted him in the first place.
Ratchet, meanwhile, with the increasingly theoretical "help" of Captain Qwark, is searching for Clank and for a way into the Great Clock. This brings him into contact with Nefarious's forces, as well as with the freedom fighter General Azimuth, the only other surviving lombax in the galaxy. While Clank learns the ropes of his new job, Ratchet and Azimuth work to find a way to get into the Great Clock, find Clank, and stop Nefarious.
A Crack in Time splits its gameplay somewhat unevenly between Ratchet, who does his usual thing, and Clank, whose levels are more puzzle-based. Ratchet's stages are fairly predictable but entertaining; one of the few weaknesses of the Ratchet & Clank series is that it doesn't really mess with its own formula too often.
Granted, that formula involves using enormous guns to destroy entire armies of robots and aliens, which is almost always a good time. The only tricky parts of the game are earning skill points (Insomniac had achievements in their games before anyone else), boss fights, some platforming sequences, and the occasional normal combat sequence, but it works here. Ratchet is supposed to be a big damn hero, so it's natural and sensible that he can do six impossible things in a row without having to try very hard. It's that balance between effortlessness and sudden challenge that really makes the Ratchet series stand out, and it's in full effect for A Crack in Time.
The counterpoint to that is that this formula hasn't really changed in six games. Even Deadlocked just emphasized the combat without actually making it feel any different, so playing all the Ratchet games in a row gives you a real sense of deja vu.
(My other mild criticism about Ratchet's game is that Ratchet takes damage in a strange way, where he only visibly reacts to being injured if he's standing still at the time. It's very easy to get nickel-and-dimed to death without even realizing you've been taking hits, and always has been.)
What helps with that on A Crack in Time is that Ratchet's game changes up its style a lot, switching from spaceship battles to hoverboot races to more traditional gameplay without much of a pause, and Clank's game is mostly devoted to puzzles involving time manipulation. It doesn't do the game's pace any real favors, as I've found myself irritated with the relatively slow pace of Clank's levels, but it makes for a more varied experience.
A Crack in Time is essentially safe. It's a Ratchet & Clank game, which means it has high production values, a good sense of humor, a high number of explosions, accessible gameplay, and much of the same stuff you're used to from the games that have come before it. You can't really go wrong with it; it's a consistently excellent series, with only a few slight issues here and there.