If I enjoy the game, or if I'm really bored. Like Levian, I enjoy the extra challenge. I'm not sure how many games I've got 100% in, because my account is bugged out and thinks I'm missing 1 achievement in every retail game, but it looks like I got at least 12 arcade games.
Big titles include GTA IV (which was the most work), Halo Reach, ODST (I got close with both 3 and Halo Wars, but I got bored on the home stretch) L4D2, BioShock (I did clock BioShock 2, but then they brought out DLC and I just could not be bothered), uhh... what else. I'm close with a couple. I've beat RDR a few times, and now I'm shaving off the last few multiplayer DLC achievements. Also... L4D1 has this whorish problem with the Sacrifizzled achievement where NOBODY can get it unless they hack their achievements, so I am missing that one.