Let me tell you a couple of stories. The first comes from a game of Werewolf, wherethe party was attempting to rescue the kid of my character from a group of corrupted werewolves. Before long, it turned into high farce. One of the party randomly stabbed himself with his dagger. His opponent laughed, then randomly eviscerated himself. A werebear ally of the party attempted to do a WWF-style grab-and-slam-into-floor, only to accidentally grab the same character and kill him.
In another, in a D&D game, the GM had a rule that a roll of 1 was an automatic failure for everything. The sorcerer melted his Wand of Acid Arrows. The fighters threw their swords around. It wasn’t very good.
I don’t use fumbles at all, any more, even in games that allow them. Simply put, the PC’s are the heroes to me, and they shouldn’t be pointlessly humiliated or made to look incompetent. "Realism" be damned, (only most critical failures aren't even realistic--above points about baseball bats) Bruce Lee or Aragorn don’t randomly hurl their swords about or punch themselves in the head, and it’s those sorts of characters who players will usually want to emulate, more so than Jack Burton in “Big Trouble in Little China”.
Maybe in a grittier game, I would, but then I'd only implement the fumble if I can come up with something that doesn’t make the PC’s look clownish or incompetent. Something like:
“Your opponent sidesteps you, and trips you into the mud.”
Your target barely ducks the axe blow, and it lodges in the tree, lodging in the bark and causing you to lose your grip on the hilt.”
See the difference? Much better, IMO, than "You fall on your face." or "You sling your weapon away.".