I have them on most of my semi-autos. Yes, there is an accuracy improvement. If you port the muzzle you'll also have an improvement in rapid-fire handling, too.
I don't just drop them in, though. I purchase the, 'semi drop-in' barrels and do the final fitting, very carefully, myself. I'm saving up, right now, to get a new 9mm Bar-Sto barrel for my Glock Model 19.