The Seiko MM does look like a dress diver but it is a tool watch at heart with its monoblock case, beautiful bezel and action, amazing lume, sweet ratcheting extension clasp, etc. It is dressier compared to the Sinn U1 but is nowhere remotely near a dress watch IMO. I think it's a great candidate for a do all and go anywhere diver.
I do see these for sale on occasion but nowhere close to the volumes of say an Omega PO or IWC Aquatimer so I don't really see this as a reflection of how good or bad a watch is.
I have not owned the MM 300 or Aquadive but based on looks, heritage, and bang for the buck, I would have to go with the MM 300 by a considerable margin. Due note that it seems that Seiko likes to be conservative on rating the accuracy of their movements (or at from my experiences browsing the Seiko forums). My only gripes about the Seiko is the hardlex and I find their bracelets and clasps hard to get a good fit for my wrists.