I went from MK-II in Zeta to MK-V in Sentinel. I didn't know anything about the MK-III until I stumbled upon it in some website and I liked it. I really liked it. It has the amazing Zeta looks that I'm surprised it's part of the MK series, probably due to the non-waverider.
The grim face and grey prototype-colors worked well into shaping a sadistic killing machine. Plus the lean and mean body and rather impressive equipment makes this an intimidating Gundam. I dare say I rank this 2nd behind S Gundam and the FA version behind Ex-S.
I think it would look amazing in RG form but given the unpopularity, I doubt there's going to be any kit of this monster. Which is a pity since I think this deserves plastic form over the recently announced HGUC Messala [No offense Zeta fans!].
Nonetheless, I paid a pretty hefty price and it is justified with the amazing feeling of owning a masterpiece. I've nothing else to say but it looks better than in pictures.
For something to look good in all angles and looks awesome just standing still, this MK-III ranks a 7/10.