I'm not saying I wouldn't rather be a starter, because I would, but being a career backup QB in the NFL has to be one of the sweetest gigs in all of sports. You play a premium position, the fans generally love you and even if you do poorly, it's like, meh, he's just a backup anyway, so you don't really get any of the wrath. Because you don't play too much, you don't take many hits, so you generally get through a career unscathed. You get to have a few glorious moments and make a load of cash.
Pretty good way to spend 10-15 years if you can do it.