Jason, I have a crown and have done some macro with that, it's quite good. Really nice for travel macro. Actually kinda light and not so stable as one might like. But anyway I normally use the Nikon 120 AMED for macro with the crown, and a diopter from time to time. Here are what I see as the disadvantages of the crown: GG composition at large mag with slowish lenses is not a pleasure for my eyes. Also there is no really convenient way to meter; bellows factor becomes a real headache. However you can go to high mag and get beautiful results, no question about it.
Now, a fair amount of the flower macros I've done involved subjects that just aren't sitting still for long periods, so I find myself grabbing the rb and happily giving up a bit of WLF brightness for the TTL metering.
Lens wise, I just don't think you can get any better than the newer RB lenses, and anyway, for macro you are often stopping down appreciably so it's kind of a wash. Having said that, the floating element rb lenses are quite a thing. My *only* gripe is that I wish they'd have put longer speeds (2 sec, 4 sec) into the macro lens and I wonder about slip-in aperture disks for improved bokeh.
I do use a 612 back with the crown, it's good. But I think if you look at the cost of a rollfilm back for 4x5... and the cost of a motorized 6x8 back for the rb....