I have two witness pistols. A 9mm and a 40 S&W. I love em, they feel better in your hand than any other pistol, have 1911 controls, are made of steel, and there are plenty of factory and aftermarket parts available to do whatever you want to them. Hi-cap mags are reasonably priced too.
I use the 40 S&W for IPSC limited competition. I have put 4000+ major power factor rounds through the 40 S&W without any problems at all. The only modification I made to the gun was the addition of a BO-MAR sight. I carry an extra slide stop in my range bag because someone told me they tend to break, my experience doesn't support that however.
I haven't shot my 9mm very much, less than 300 rounds. I was considering making it into a 9x21 for open IPSC competition but have since changed my mind. I now wish I would have bought a 45 acp witness instead.
I do have one minor complaint. I shoot primarly lead bullets (Oregon Trail) and the barrels seem to strip/shave lead. After shooting even a few lead bullets you can find lead shavings in the barrel-looks like hair. It doesn't affect accuracy or reliability. After 300 rounds (give or take) I just shoot a couple of jacketed bullets to remove the lions share of lead at the range then finish the cleaning at home.