All the RIP games are fast, intense overhead-view third-person shooters with hordes of enemies to destroy and pretty nice sound and graphics--a lot like Crimsonland, in other words, which is by no means a bad thing.
Each iteration has improved on the gameplay of the previous game. The original RIP has you confined to a turret on each map--with the geography of the map and the paths taken by by attackers used in the levels' designs to create interesting dynamics. RIP Strikes Back added the ability to leave your turret, with additional refinements to the character levelling system to provide temporary power-ups, as well as boss battles. RIP 3 adds a huge number of new weapons, vehicles, and destructible terrain.
One odd (and curiously charming) element of the first two games was that you had to chose one of three characters to play: Death, Halloween, and Rock 'n' Roll, all pretty bad-ass, but kind of a peculiar idea on the part of the designers; each had his own advantages and drawbacks. In RIP 3, you can also choose from a variety of merc-like characters, if you find the original cast too silly.
Fast, gory third-person shooters were once a mainstay of PC gaming, but rarely now appear in the conventional market; some have done very well in the indie market, however, and the RIP series is among the best.