I have the 105-210. It is sharp and reasonably contrasty. I use it with high contrast slide film mostly, such as Velvia. A good tripod is a must. I would not compare this lens to the prime lenses in my Mamiya kit, but that's not too bad since I think the Mamiya lenses in general are great. For 645 I don't think you will find a comparable zoom, so if you can find one at a good price I would go for it.
Bob http://new55project.blogspot.com
I have a 105-210mm ULD zoom lens also and although I haven't checked it out that much it appears to be sharp, I'll know soon as I'm developing some film shot last week with it.
I like the lens and the Mamiya quality is built into it, the ULD version, if there are any other versions, is outstanding. Generally I don't work with zoom lenses but in this case I came across one that was new for a hundred dollars and thought why not. I use mine on a tripod.