I've started playing with it recently. Its very good, but of course it has weaknesses like everything else. The reason I like it is it works well gi and nogi, It is an effective way to get into leglocks if someone isn't standing and it is submission oriented. I mostly sweep and then attack from the top, so K-guard gives me something a bit different. One thing I've noticed though is I really have to speed up to play it. I thought when I saw it I could just sit around and wait for them to make a mistake. In my experience as soon as I get into it people will react and force me to try a submission. I haven't tried it the way Melanson does it, so maybe that would help slow things down a bit.
Also I believe the Miyaos also play K-guard quite a bit nogi. I got some ideas from them on how to play it.