I've had one on my G20 for 15 years or so. I usually use this as a night stand gun, and my elk hunting side arm. I've noticed about a 50-75fps velocity gain from it. Naturally, the biggest boost is with the slower powders.
I still CC my G20 and am too lazy to change the barrel to do it...yes, I carry my G20 with the 6". It's really not that big of a deal... it's already a big freakin' gun - another 1.4" of barrel length doesn't matter to me, and I'm only 5'10" 150 lbs.