Don't use them in my face to face games. Don't like to use them in my play by post games (though I use a variant standardized method of crits that's less deadly but still rewards the player for rolling high). I don't like them, seems like a pointless extra roll, that really shouldn't be necessary.
At my table a crit is a crit is a crit. No confirm roll (though I'll require a confirm to determine if I should pull from the fumble deck instead of using our old method), makes things more deadly at 1st and 2nd level but once the PC's have a nice HP cushion under them around 3rd things tend to even out. Especially seeing as they can crit the baddies just as easily as they can be critted. Considering I run from modules the badies aren't built as powerful or as min-maxed as the PC's.
In the end it balances out, don't see it even making anything more or less deadly. PC's die as often in my games as they did in 2nd edition. Were I to have a new group that whined about it then I would, at the very least, consider a nat 20 an auto crit.
I and every 3.x gaming group I've ever played in or DMed for has used them. Statistically, the confirmation rules favor the PCs over NPCs. The DM generally has a greater chance of inflicting critical hits against the PCs (as he/she makes more attack rolls than the players), so without the confirmation rules critical hist become more frequent and PC mortality (and the chance of TPKs) increases dramatically... and that can really put a damper on storytelling and role-playing.
I'd really like to see these statistics.
Most monsters I've seen require a nat 20 to crit. PC's, more often than not, have at least a 19-20 if not wider crit range. Some class based NPC enemies might have a wider range but they're built on a law of averages rather to the specs and standards of most PC's I've seen (in other words, they're hardly as optimized as a PC will be), so I would really like to see the math and statistics showing that not having critical confirmation rolls would favor the DM over the players rather than having critical confirmation rolls...
Does your group not see more 18-20 or 15-20 crit ranges? Is it because you have a gentleman's agreement not to use high crit range weapons?
Occasionally someone will make a build like that, and they'll crit quite a bit, but often it's with a weapon that doesn't do a terrible amount of damage in the first place (keen rapier for example). If anything a high crit range weapon just makes a fighter suck a little less, rogues become a little more deadly as they're more likely to crit on a sneak attack without rolling a crit confirm check but even then it only modifies their weapon dice.
Could be the party dynamic, the guys who dish out the most damage in my group are often spellcasters, or some kooky rogue build where an extra d6 of damage + str bonus really isn't much of a thing. My players do min-max a little more than most I guess (not that I've taken a survey and quantified how much the average 3.x gaming group min maxes compared to mine or anything :P) but toss in slightly stronger monsters or altered NPC's and it balances out once more.
That and if they roll a miss they still miss, a nat 20 is the only guaranteed hit in our group.