Nick, the issues I had were with the rb 50. It had significant distortion and wasn't as critically sharp as I had hoped, especially in the corners (although, as I noted elsewhere, I was thoroughly spoiled by a 50mm on a mamiya 6 at the time I reviewed the rb 50).
The rz 50 ULD is very good. But if it's out of your price range... well, I now have an rz 65 L-A and I like it very much. It is a very sweet lens at a good price. If you don't really need that extra few degrees of coverage, the 65 is what I'd recommend.
For landscape and scenics you might be 100% satisfied with the non-ULD 50. For things like architecture, forget it... in my opinion.
Overall though... for me the 65 is tops.