Never actually done the MM model but have done loads of JBs over the years, often with the Marcus sound in mind. Biggest single factor for me was the maple fingerboard. The various other factors seemed to be pretty marginal to me but the maple board had quite a transformational effect given reasonable amp, cab etc.
My current MM variant is my old MIM body (no idea what wood this is), maple JB stock retrofit neck with maple board plus barts and badass all run passive into an aguilar OP3 that I've built into an external stomp box so I can use it with different basses.
I am now told that the only obsticle between me and Marcus is talent.
I like the pickguard although I've gone t.shell myself. Front pickup cover on which I did actually find to be helpful for thumbs, not so good for fingers if you want them over the front pu.
At some stage I should really come up with some ideas of my own.......
As far as the MM is concerned the prices have always put me off a bit. I was wavering a couple of years back but in the end just went with my own take on it. I've heard great ones and indifferent ones since, not to the degree though where I could conclude it was a good or bad bass as such...