If I had one I would. I really like the Kmp9. I run a DMR so its a little much to carry around for me, buy if I had a bolty, it would definitely be my sidearm of choice. Don't know how I'd carry it though.
I was thinking of running a KWA KMP9, I ended getting a Tm 1911, but my friend purchased the KMP9, and it's suprisingly light! The mags feal heavier then the SMG it self. The rate of fire and accuracy is pretty outsanding, especially witht he short barrel it has. You'd only really need 2 mags if you were using it as your sidearm, also note that the mag release is super tough; making it optimal to crawl around with, as you won't loose the mag.
I have a KWA mp7 and I love mine! It's a great weapon and I would say its about perfect as a secondary (Although a bit high on the fps 420). It's not really a problem for me though. The range on it is incredible for such a compact little gun and on full auto it can really scare the crap out of people cause this baby is loud!!