The only game I can really class as a rip-off was Beast Wars for the PS1. Bought it because the show kicked ass (and because I was about 9), and it was horrible beyond belief. Terrible graphics, even by PS1 standards, clunky gameplay and a difficulty curve that's best desribed as 'hitting a brick wall once the level loads'. Ended up selling it for £2.50 to a local shop about a year after I bought it (I tried so desperately to like it), but now I regret doing so, it's the only time I've ever traded in a game, well except a duplicate copy of GTA2, but that doesn't count.
Apart from that, I've never bought a game I deemed a rip-off, even things like Too Human, Deus Ex:IW, Black & White 2, The Simpsons games (Virtual Springfield, Wrestling, Road Rage, Hit & Run), because I got enjoyment out of them all. Seriously, Beast Wars is the only game I've ever bought that I literally could not bear playing.