I dropped a Minolta lens onto concrete from four feet. It warped the mount (fixed with gentle filling) and messed with the aperture. It was incredibly stiff and just not right. So I got out the screw drivers and attacked it. :mrgreen:
The workings are pretty simple (you can't, or rather I didn't try, to get to the actual workings inside the lens) so if you like to take things apart you have the perfect excuse. Just be careful not to lose any of the bits and remember where each part goes.