I just like it when the achievment pops up at the bottom of the screen,that is, as long as it's for something awesome, instead of just beating a chapter. Then I have acknowledgement that I am awesome, and as an egotist I very much enjoy that. (ICortezI is not acually an egotist, this was actually a clever joke that in hind sight wasn't all that clever.)
I also like competing with my friends to see who can get the hardest achievment or who has the most points, it's not all that important, but it's a nice, fun, pointless tack on, which video games are full of, and not all of them are nice or fun, so I think they're alright.