I had one. Real good gun, well made, good materials. One or two sharp edges out of the box but nothing 5 minutes with emory cloth didn't solve.
Reliability was good. Had a few FTFs in the first 50 rounds but after another 50 and a good clean and lube, I had no futher problems.
What I didn't like about the gun is despite its small size, it was heavy and a little wide. I bought it for something smaller and lighter than my Kimber Compact that I normally carry and it didn't really fill that need.
Not much fun to shoot, either. The short backstrap concentrates a lot of energy in a small portion of your hand - Corbons and Tritons are nasty; that said, it's not a recreational gun and it's a lot of power in a small package.
I did eventually sell it and got an STI LS40. That gun is thinner and carries better for me and being a single action and a near 1911, it's much closer in feel to my CCW and competition 1911s that I handle so much. The STI is significantly softer shooting too.
The MK40 is still a good gun though, if its what suites you. I wouldn't hesitate to recommend them to someone or even use one myself.