I really, really like the concept. The idea of putting in little rewards for being creative, playing a little longer, or going someplace you'd never normally go, all work in tandem to increase the gaming experience.
In practice....it can be a double-edged sword.
On the one hand, hearing that cheery little 'ping' always makes me happy, whether it's intentional or not.
On the other, some are so hard as to be downright nasty. The "Flawless (insert instrument here)" Achivements in Rock Band just pissed me off to no end, and I still need the drumming and singing ones. Not from lack of trying.
If you're really obsessed, then you fret about not getting some obscure Achivement that no one else cares about. That time could be better spent playing newer, better games.
And sadly, I fall into the "Fairly Obsessed" category. I've got nearly 6000 points so far, and I really try hard to get as many as I can. I will admit though, it's pushing me hard to be a better gamer. My Halo skills have sharply spiked from trying to get the multiplayer Achivements, to the point where it was me vs. my friends and I still won 75 to 50 and down.
So, I like them for the chance to get more out of a game that you wouldn't normally.
However, I do hope that my gamerscore never hits like 100,000. That just screams "No life!" in my opinion.