My 20 rounders work fine, but it seems to me that they disturb the balance of the pistol a bit. Not so noticeable with a Brig Tac (perhaps because of its heavier slide), but a bit irritating with an M9 in my experience. However, my guess is that this is an individual preference thing.
The 18 round Mec-Gars work well, as do the 15 round Beretta mags and the new (non-parkerized) Checkmate mags (heresy, I know, but I have found they are perfectly fine range mags). One of the best points of the Beretta 92 series is that mags tend to be cheaper than for other pistols these days.