Karl Rohrbaugh stands behind his product. Any manufacturer of any product has some unhappy customers, as much as the feedback is mostly positive on the Rohrbaugh forum, occasionally someone has a problem and posts.
The advice is alway that they send it back to Rohrbaugh, and in almost all of the cases that I've read about Karl fixes whatever the problem is.
I haven't had a problem.
I practice everyday with A-Zooms, doing magazine changes over and over so I can get good at it.
The gun has a heel magazine latch to eliminate the possibility that the mag release gets bumbed while its in the pocket. Some people don't like that. I know it makes for difficult mag changes, so that's why I practice swapping out magazines. I've cycled hundreds of A-Zooms and Tipton Snap-Caps through the pistol just practicing mag changes.
As you travel up the scale you can always say one or two or a few onces doesn't make a difference. The MK9 is slightly smaller than the PM9/CM9 but it's a tad over 22 ounces empty... I think it's a great little gun, just not a pocket gun for all pockets. The PM9/CM9 is close to the R9 in weight, but slightly larger - longer and higher, the R9 is 5.2" long, 3.7" high. The R9 weighs 13.5 ounces empty, my R9 weighs 18.3 ounces loaded with 7 rounds of 147gr Gold Dots.