A while back I decided to try using d6 for everything in classic D&D. I already used abstract damage (everything was a d6 although I gave twohanders 2d6) and I used d6 for hit points as well, with Fighters, like the ye olde Ranger, getting 2d6 Hit dice.
Now I have heard people mention using 3d6 in place of d20 in the past and I was eager to try it. I've had many a game night mellow out very quickly because one player could not roll greater than 10 on a d20. If you're familiar with the math behind the latest two incarnations of D&D, less than 10 is usually a death sentence unless you've walked into the realm of absurd min-maxing synergistic munchkinism. Well, perhaps that's a bit extreme but it's something I wanted to avoid. Which is why I was drawn to 3d6 with it's wonderful, truly wonderful bell curve.
I've been using 3d6 for quite some time in my Labyrinth Lord game and I have had zero complaints. The only time it would come into conflict would be at low levels, where an 18 saving throw is likened unto instant death, to counter that I allowed certain Stat mods to be added to certain saving throws (Dex vs Breath Weapon, Con vs Save or die, and Wisdom already applied to Spells) and fortunately since I had a Dwarf and Treasure Hunter their saves were a bit beefier. I imagine this would be a problem at much higher levels as well when a Fighter's saving throws are in the realm of 4-6. This won't be a problem for me in particular since in my games there is a soft-cap at level 10.
I like it enough that I've been testing out using 3d6 in the game system I'm working. Things have been going well so far but in our most recent playtest I've come to the conclusion that I will need to get rid of ability modifiers. I've managed to solve most of the problems that would occur in doing so but that's time better spent in a later post.
I should note that I still use d100 for Thief skills.