I've used the Mamiya 120/4 A with a Mirex T/S adapter on a Canon body for a few years now. No complaints from me. I use it mostly for automotive 'detail' shots from fairly close range but rarely 1:1.
The lens is very sharp wide open which is how I tend to use it, for shallow DOF, but it does sharpen up further at F8 where it is extremely sharp to the edges.
This is at F4 with NO tilt at all, the bokeh is quite OK at close range.
These are at F4 with tilt to control focus along the length of the car. The lens is great for that kind of thing.
Macro with tilt is problematic because you need to increase tilt as you increase the magnification ratio so you get to a point where it just isn't very workable any more because you need more tilt than the adapter/bellows/view camera or whatever can offer or that the lens can handle. Your own application might determine how useful this setup really is to you but for typical product shots it's a really nice setup, easy to use and reasonably cheap too.
There are other ways to achieve tilt/shift for macro work, such as the Nikon PB-4 or a few other similar macro bellows. The PB-4 might be the cheapest option with a decent lens like a Schneider 100/5.6 Componon-s rounding out the setup.
The PB-4 also gives quite a lot more tilt than adapters like the Mirex, I think it's about 25 degrees, but don't quote me.
