Dear John and William,
By all means, get the 120! I am a professional nature photographer and the lens is superb. I own the 645, with 35, 45, 80, 120 and 210 lenses. The 120 is the lens I use most, but not just for macro. The sharpness and color rendition of this lens are just unbelievable for landscapes as well.
You can see images with this lens at my website
www.guyharrisonphoto.com. My images do not carry technical info, but most of the "intimate world" photos were with this lens. Also, check out the Yellowstone sunset with geyser and bison in the western landscapes gallery--again with this lens. You want sharpness? Check out the totem image in the Alaska gallery, it doesn't get any sharper than this--every grain of wood shows.
Best of all, the lens still gives high quality at f45, where I shoot all the time.
It is slow to manually focus, with a beautiful but heavily d&ed feel--but remember this is a specialty lens, and slow, deliberate changes in focus are the norm for macro shooting. I rarely use AF in medium format so this is not a big drawback for me--but portrait or wedding specialists might prefer the AF 140 mm model, with its faster aperture facilitating handheld shooting.
I recommend this lens without qualification as to its image quality. You will not be disappointed!
Guy Harrison