I like fumble rules- if you can critically hit, you should also be prone to fumbling. If a PC rolls a natural 1, I have them make a DC 15 Reflex save, and if they fail, they fumble. Here are my tables:
Result
1 Drop weapon
2 Damage weapon
3 Provoke AoO/Distracted, no further missile attacks this round if missile
4 Hit target nearest intended target (roll separate attack roll)- if no other target near, roll attack on self
5 Off balance: Suffer -1 AC until next action
6 Move 5' in random direction due to momentum of attack
7 Trip self
8 Pull muscle, -1 on all physical activity (attacks, skill checks) for next 1d6 rounds