Bowser on these forums owns a KWA Mac 11. I've seen him use it on the field before, he even got me out with it once. Thing's pretty sweet, packs a punch. He's never had much of an issue with it to my knowledge.
As for the above comment on full auto GBBs...
For gas blowback RIFLES, they can be a little impractical, seeing as they tend to hold no more than 30-40rds per magazine, and are easily outgunned by most AEGs. Of course, what you gain is an immense cool factor. However, many full auto GBB SMGs, specifically those made by KWA, are great guns as lightweight primaries that pack a punch. I've heard great things about KWA's MP7, MP9, and M93R. Of course, just because your gun is capable of full auto, doesn't mean you need to use it. However if you choose to, it certainly has a way of getting enemy heads down on the field. The rate at which one burns through ammo is entirely dependent on one's play style and trigger finger, not the weapon which they wield.
NOW, all that said, while I agree it is tempting to buy the Mac 11 due to it's cheap price (cheapest gun in KWA's lineup), I have heard of different issues popping up with that specific gun, as well as it's sub-par accuracy (to be expected though) and extremely limited customizability. My suggestion would be to save a bit more and look into the KWA M93R, or even the MP9 or MP7 if you can afford it. Depends on what exactly you want it for though... decent sidearm? Lightweight primary?