I am with Whizbang, here. I absolutely hate them and never use them in games I run. In games in which I'm a player, I'd gladly offer to give up the chance to ever crit in exchange for immunity to fumbles.
1) They are far too frequent. I have fired hundreds of bullets and hundreds of arrows and never once broken a bowstring, jammed a rifle, or shot myself. Perhaps at 12 years old I was just that incredibly skilled?
2) They are far too severe. Weapons break WAY too easily on most fumble charts I've seen, compared to how hard they are to break with attacks.
3) At mid-to-high levels, the auto-miss on a 1 is ALREADY significant. In many cases, the only way the fighter can miss is if he screws up. Oh look, he rolled a 1 and missed. Guess he screwed up. No need to make him look stupid on top of that.
4) They make the characters look pathetic and incompetent. In my mind, a level-1 fighter is still a "veteran" as in BD&D -- a trained soldier. The only morons who hit themselves with their own weapons in a cinematic scene are the frickin' Ewoks in RotJ. I don't want to play an Ewok.
5) Being disarmed or having your weapon broken should more often be the results of actions than accidents. Stick these on the fumble chart and you make Disarm and Sunder basically useless. Just go full defense and sooner or later your enemy will disarm himself.
6) Critical hits are already balanced by the enemies having critical hits. However, they aren't particularly necessary and the game would be fine without them.