The Smith brake is extremely effective at keeping the muzzle level under rapid fire.
I had a Daewoo DR200, which ejects a jet of gas stright down on firing. In stock form, this would quickly turn the rifle into an anti-aircraft gun when firing offhand, but after a little "surgery" to install a Smith brake (the model for post-ban heavy barreled DPMS rifles), the muzzle stayed dead-level.
I've had 2 on ARs, also. One was a pre-ban type, and the other was the set-screw post-ban type. The Set-screw type generated a MUCH smaller flash then the pre-ban model did. I think maybe the interior volume of the brake has something to do with it, as the post-ban brake is much larger in diameter. The One on the Daewoo is also a post-ban model, and it generates almost no muzzle flash at all. Neither pre-ban nor post-ban model sounds louder than the other; they'll both draw looks from people wondering who is firing the cannon. (although actually, a 155mm howitzer shooting a high charge is not as loud, truly---ask me how I know)
The BM AK74 type brake is also very effective at keeping muzzle climb to a minimum, and also directs flash straight to the sides, keeping it out of the shooters' view. The one thing I wish BM would improve on the AK brake is to machine the bottom of it at the main ports much wider (as the original AK74 compensator is); this would keep the dust signature down.
Between the Bushmaster AK brake and the Smith compensator, I believe the Smith to be a little louder to the shooter, but that's subjective.