Thought I'd chime in as a very happy KRISS owner. Is it a gun that one 'needs'? Probably not. It is, however, very fun and the lessened recoil means my wife loves it (and she's tiny).
It does use Glock 21SF mags and the 30 round extensions make it well worthwhile.
Availability depends on whether a local gun store has become certified as armorers for it. Here in KY, only one gun store in the state got the nod (and thankfully they are in Lexington where I live).
One of the earlier posts mentioned we never see things becoming LESS complex. I have to say the KRISS is very easy to take apart (few pins and you are done) and the genius of the thing IS its simplicity. Believe me, if I can handle taking it apart and putting it back together, anyone can do it!
The downside is that it is a pricey investment. However, it always turns heads at the range and people love shooting it. I was most surprised by how accurate it is given the 45acp round. Given that, I can take it to the indoor range here since it is still firing a handgun round (can't do that here with other evil black rifles!).
Happy to answer any questions, but don't overlook it if you get a chance to see/fire one.
- Ken