Achievements have lost their appeal to me a bit, in recent years - as is demonstrated by my 980/1000 on Lego Batman 2. There's one achievement remaining - play a level co-op. Could easily do it with one of the kids, or even by turning on the second controller but not using it, but I've finished the game and done everything (apart from a co-0op level, it seems) - the game has gone to the shelf, and not many games make it back from there.
They're useful, though, on occasion, as they direct you toward things you might never have tried or noticed ordinarily. I like it when you finish a game and see there's an achievement for saving a certain person, or an achievement for seeing both endings, something like that, which makes you consider replaying a game to see some things you missed. Without those achievements being there I might never have bothered to replay the game, and would've missed out. Of course that only applies to games I enjoy. If I don't like it much I ain't replaying it to get the second ending, no matter how many achievement points you're offering me.
I generally can't be arsed with going out of my way to do legwork to get extra achievements though, once I consider a game "finished". Life's too short. I've a big stack of games I need to play - I'd rather put those in the machine than replay a game I've completed twice over just so I can find the final hidden object or two. And online multiplayer achievements can kiss my arse.