So, here's what I know so far:
1) The mag-well is tight. But, it worked well with my 10 round RRA mag. At first the mag didn't drop freely, but with some very minor and strategic sanding of the finish on the mag, it drops free and clear.
2) It seems to be compatible with aftermarket pistol grips. Danielle, at Wolverine Supplies, installed an Ergo Suregrip on one of their MR223, to verify for me. I promptly ordered the Ergo grip. I also ordered a ARMS #40L BUIS, and Midwest Industries flip-up front sight.
3) HK has three different pistol grips available for the MR223. Wolverine Supplies will be able to order them when the new price list comes out in a couple of weeks.
4) HK also makes different butt-plates for the stock. I am also pretty sure that the stock is interchangeable with aftermarket units.
5) Like I said before, the fit and finish is absolutely sublime. You'd really have to see it and handle it to know what I mean.
6) The trigger is really outstanding. It is lighter than the trigger pull on a Swiss Arms, and breaks clean. The trigger on my HK SL8-5, on the other hand, has a spongy trigger that feels like ####.
I've been through a bit of an emotional roller coster with this gun in the past 24 hours. My initial enthusiasm was soured a bit by some of the negative comments. But, after a bit of research about potential accessories, and after spending a lot of time examining it tonight, my enthusiasm is renewed. I am going to play hookey from work tomorrow and put a few rounds down range.