Well, really there are alot of things about rip-offs that can teeter in on the good or bad sides.
Saints Row 2 is an example of rip-offs done right, as it's made in such a way that that it actually does have some originality. Much of the gameplay is different and some of the story is different, but looking around the town, looking at the character, and just about anything else you'll see rip-off, but looking at the rest of it, it has alot of originality.
Darksiders had a very different plot that was at least interesting, and did rather well for the most part. Heck, even finding out some bits and pieces at the end made it rather interesting even if you had to raise an eyebrow. The 'mash button' technique was also somewhat original, though not quite, but the major rip-offs were the weapons and the puzzles. I somehow was able to dodge the thoughts of how similar most of the weapons were to other games, until I got the voidwalker, which they didn't even change the colors of the portals to make it appear differently. Darksiders had alot of originality, but some of the rip-offs they made were too well exposed to be ignored.
Dante's Inferno. Fuck..really, that could have had more of a rip-off and I might have enjoyed it more. But the fact that I felt like I was playing God of War with an uglier man who happens to be a crusader (which crusaders to me are what Nazi's are to Yahtzee for the most part), is a little disappointing. The idea was....okay, but the fact that all the wrong parts were exactly like God of War, and any parts that I would have wanted them to rip from GoW, they didn't. For crying out loud, if it takes me 20 minutes to kill all the monsters in the room, then there's really no reason for it to exist x.x. At least with Kratos you could walk along a stupid hallway...
EDIT: But to conclude. Rip-offs aren't all bad, BUT, you need to do at least one thing to make it original, even if you have to have the main guy with a very long nose or antlers to make it original.