Despite my previous post, I've rarely/never actually used critical hits or fumbles in my own D&D games (though some DMs I played with did) -- 10% chance of a crit or fumble is too often, the alternative (adding additional rolls to confirm) is too cumbersome, plus Gygax railed the entire concept in the 1E DMG and I figured (rightly or wrongly) that he knew better than me.
I did however use (and like) critical hits and fumbles in RuneQuest, where they're based on skill (lower-skilled are more likely to fumble and less likely to crit, higher-skilled folks the reverse), occur less often (roughly 5% of the time, instead of 10%), and the fumble tables are more sensible than a lot of the house-rule D&D tables (most fumbles result in losing the next attack or at worst a dropped or broken weapon -- in all our years of playing RQ I'm pretty sure I never saw a single PC and only 1 or 2 monsters actually roll one of the really dramatic fumbles and chop their own arm or leg off or accidentally kill a friend). I liked the crits and fumbles in RQ both because it added to the sense of "realism," emphasized the unpredicatbility and danger of combat, and also because it helped draw a distinction between the experts from the mooks -- a swordmaster with 120% skill crits on 01-06 and only fumbles on 00, whereas the mooks with 30% skill only crit on 01-02 and fumble on 97-00 -- which is a more important consideration in a game without D&D's dramatic power-curve of hp, AC, and THAC0/BAB progression.