I had a Colt 9mm AR before I went on one of Geauxt's infamous boat rides. Mine wasn't the SBR though. It's a nice rifle but if I was going to get just one AR in would get the 5.56. If I'm not mistaken you can convert a 5.56 to a 9mm with a new upper and a block in the lower. Some of the other AR experts on the board could provide better info on the conversion.
I was thinking about a 9mm AR but didnt see the point in spending that much on a dedicated 9mm upper. I bought a cheap arse Hi Point 9mm carbine and its been fun as hell. I would not trust it for HD but its a perfect truck gun.