I've had a Retroachivements account for ages now, but only had a handful of achievements on it. After seeing this thread, I'm going to make an effort to 100% some of my favorites. Aria of Sorrow looks to be about what I expected, although there are a few you have to do on the secret modes which I haven't done before. Sonic Adventure 2 likewise is fairly similar to the Steam achievements, although there's a lot of "find all the animal" achievements (not terribly interesting, but not difficult) and "beat every boss without getting hit" (going to be a massive pain).