I complete most of my games, with exception of those I think are horrible (N3 forexample). I also do a lot of them up to 100%. Not that big a fan of online achievements and such though. Many people may say that focusing too much on achievements or such will ruin the game experience, I solve that by doing first playthrough for fun and then usually take a second playthrough where I focus on the achievements I don't already have. To be honest, it kind of annoys me watching friends play and just boost through a game, not taking their time to look at things happening around them or bothering with sidequests.
But I do know a lot of guys who rarely go for the 100%, and a lot of guys who only completes the games they love the most, which usually isn't more than 3-10 games.