I agree with scott that a tangible reward is always better than just a pop-up. That said, I do think that achievements extend the life of games and help you to sort of figure out how to push the game to its extremities (if done correctly) when you aren't hardcore enough to obsess about finding every last thing you can do. A lot of action games have fun ones like "get a 10-count air juggle combo" or something like that, and while you might do that without the achievement, then it's just... hey, I did something neat. If you set a milestone for it, it can become a goal, and games are all about goals. So yeah, I think they're good things as long as the goals are fun and worthwhile, but any achievement will pale in comparison to an actual unlockable item of some sort. A combination of both can be good, too.