My girlfriend has a 2009 13" MBP. The hinge is a little weaker than it was new. When the screen is open less than 90 degrees it feels like it will fall closed with a VERY light push as opposed to the "normal" amount of pressure. It is ok when opened beyond 90 degrees though, and it doesn't seem to move that much when typing on it. Of course this is with a 13" and has less weight to support. And she is pretty light on it too. So they last ok, but don't expect it to be like new in a few years.
Not sure how hard the whack is, but I have a MBP 13" that has been dropped on hard concrete on a rainy and slippery day. The fall was so hard that a part of the display assembly was bent permanently, and the hinge was still fine... for a very long while after that.