Depends on the game, really, but I have finished nearly all games I have, but not seen/explored 100% of the content.

I have been known to play through a game over and over (reaching about 8-10 times for the Fallouts ... that's each, not together, and I only count 1 and 2 here) until I was satisfied that I had seen and done everything there was to do in the game (sometimes playing through a few more times for good measure) ... while at other times, I would throw the game away without playing it through once (Black and White being a prime example in this category, I simply grew tired of the game).

Though I generally prefer to discover everything in a game, most of the time I simply can't be bothered for a variety of reasons, from lack of time, to a dislike of some choices presented in games that simply prevent me from continuing down a certain path.

Personally, I blame my peers for this behaviour.