I'll try and complete 100% if there is some benefit, like a secret ending or something (ala KH1 and 2).

But majority of times, I do what I want until I'm bored or get stressed or buy a new game. Last 2 games I've bought though I've not completed.

I'm back on Crash bandicoot 2 for the PS1 and at 94% and intend on getting 100% (without guides), I've never been bother to go about 90% before.

 Did you 100% Sunshine? That was the hardest game I ever completed 100%.

I have never set myself a goal to finish the game 100%, but I think I have with some games.

Nah. I don't really care about optional content. I'll normally just blast through the game's main areas and call it a day.

I almost never finish 100%, but always get close.

Ocarina of time: I'd get all the heart pieces, collectables.... but not all the skulltulas.

F-Zero, I'd beat everything apart from the staff ghost time trials.

Mario games I'd tend to do 100%. It depends how minor the final challenges are. I try to complete the significant gameplay elements.

 Am I missing something? I thought you were a fellow Gamecube fan. Why didn't you buy it?

Hell no it's not linear. Think Mario 64 with a badass flood pack.  

Well, what's an example of a game that's easy to complete 100%?

Otherwise, yep. The busy life of a grad student doesn't leave much time for finding and completing optional content. There are times when I do play through some extra stuff - I got all of the optional stars in Super Mario Galaxy, not including the purple coin levels, for example - but those are usually only when that content is both easy enough to find and as interesting as the main content.

Not really . I usually beat the story-mode and than unlock the easy extras ... than comes another game :P

360 Achievements makes gives gamers a sense of accomplishment.