It was asked: "How do RB67 lenses rate, good one's bad one's?"
Any of their Sekor C lenses, and especially the K/L lenses, are superb... I don't believe any of them are bad. Another commented they didn't like the 50mm... but personally it's one of my favorite lenses. It is a little bit more unforgiving than the 65mm and takes some forethought to use it due to the ultra-wideness of its images, but it's tack-sharp as most of Mamiya's stable of lenses. By the way, my favorite of all their lenses is their 75mm f4.5 K/L Tilt/Shift lens. It's big, it's heavy, it operates differently than their others (lens has a separate mechanism for cocking the shutter unlike all their others), but the length is moderately wide, it's arguably the sharpest lens they make, and the tilt/shift function enables one to correct perspective similar to large format cameras.
On Mamiya lenses, I'd be sure to buy only lenses rated VERY high (Excellent++ or Mint), as there are lots of older ones that have been abused and/or used very hard. I also would only buy Sekor C or K/L variants, and not the older uncoated Sekors.