After 7 or so years of being a backup QB, Matt Cassel stepped into a good (no longer great) New England Patriot team and won ELEVEN games.
Guys, he's not the cure-all answer some of y'all were all hoping he'd be, but he's also not the majority of the problem. He has shown that with a good (not great) team around him, that he can be as good as any QB in the league. It's REALLY tough to succeed when you're set up to fail with a horrible O-line, a WR corp that loves to drop passes or deflect them into the air to be intercepted, no QB coach, no OC, and a head coach that does a great job of making sure you get to play from behind the entire game.
Besides... show me a better option. There won't be a franchise cornerstone QB in free agency OR in the draft.