I belong in the camp with the people who compulsively have to finish a game they started. I can only remember one time where I dropped a game without finishing it: Splinter Cell: Chaos Theory. When I encountered the first bug that prevented me from completing a secondary objective, and later while researching its known issues I found that there are occasional bugs that prevent you from completing even more objectives, I dropped it and sold the game on eBay.
That being said, I can also probably cite one or two times that I left games installed without touching them for a month or two just because I hit a spot that was boring and didn't make me wanting to go back to the game for a while. That's the curse of playing a lot of games during the same period. As soon as you hit a slow spot, you spend your gaming time on the more interesting ones. But even in those cases, at some point in the future I will go back to the game and finish it because I'm too curious about how they end the stories. I'm very story driven when I play. It's like watching a movie or reading a book. Even if it's bad, I get this weird compulsion that makes me NEED to know where it goes and how it ends.