Sometimes I find the concept of fumbles amusing... a bit of slapstick. Other times I like the added tension and drama that it brings to a story... that "oh crap" moment.
I play games for the story more than for combat simulation, and I've never had a DM that's out to kill the PCs. So fumbles don't really have any negative impact on the game for me.
But I have never been a fan of a 1/20 chance of fumbling. It would have to be more rare than that, otherwise it would just get annoying.