I don't really care about Steam achievements. For whatever reason I really enjoyed the pop up sound and the gamerscore of achievements on Xbox. Overall though I think that it's an extremely relative subject. Steam Trading Cards on the other hand....
I believe achievements are pointless. Its basically there for bragging rights. Back in the day you didn't need that crap because beating a game was enough. If a game was good enough you would play through multiple times. Now people play through crappy games numerous times just for achievement points. Hell I've seen people get a Hannah Montanna game just because it was easy to get all trophies.
They give me goals to shoot for, whether they're challenge runs or completion goals. I'd do that stuff on my own but some structured milestones are always nice.
It's also fun to go back and check out what I've done in a game and what I never got around to doing. Especially when I see an achievement I thought for sure I'd get but stopped short. Just makes me want to go back to that game for just a little longer.
I think too many gamers rag on achievements. They're a nice touch. Not a big deal.