One point about them - they do make people play piles of crap they wouldn't normally touch with a barge poll.
Personally I only play games that I like and I think are good, and although I like achievements, I wouldn't have some crap like Avatar: The Last Airbender, or some NBA game on my GamerCard, just because you can get 1000 points quickly.
If there is a negative with achievements, its people who collect them, and play utter tosh just to get the points. You wouldn't have looked at these quality of games on any previous console, and really, no-one is going to think you are 'cool' for having a lot of achievement points, are they?
You can say 'I have no taste' with your gamertag, by clogging it up with rubbish, but you can't say 'I iz cool' by having 19 billion points.