Personally.. I never stick with SP games, play a level or two and thats it. I got through bioshock though - trainers ftw!.... CoD4 had to be the only game I ever finished in SP without a trainer and even then it was because I had it on the PS3...
But as to bioshock, the ending was short yeah. But my understanding is that they are making a sequel and thats why it was short. No idea if thats correct though.