hi Dawid -
It is a great lens. I have the RZ67 version, and really enjoy it. It's not without some flaws though. Right at the edge of the frame there is a bit of smearing and chromatic aberration, which is just part of the inherent design limitations. (I've had 2 of these, so it's not just a particular copy that does it.). Still, excellent and very sharp. Stopped-down to f/8 or better, it's as sharp in the center and with as much contrast as the ULD 50mm for the RZ, which is saying something. The close-focus capabilities are crazy, you can get down to focusing on things actually touching the front of the lens. Smearing and aberrations get pretty rough around the edges though when it's that close to the subject, but to be expected I suppose.
It is a heavy gorgeous beast, no question. The front element is so wide and deep it looks like you could go swimming in it. ;-)
You can also put it on a 4x5 and get round circular fisheye images too. Usually that means removing the petal hood, but that can be done with a spanner wrench (e.g. removable and replacable front section).
Price-wise usually the RZ version goes for something like $600-1000+, depending on condition. RB versions are usually almost as much. Partly due to rarity and also partly due to the RB version also being pretty easy to use on larger format cameras (due to manual built-in shutter vs. electronic shutter in RZ version).