The non-floating versions are OK near infinity (e.g. Matt's example) but become very poor at closer focus. I found that I could get better resolution from 35mm film than I could get from the non-floating RZ67 50mm, which is a bit sad considering the 4x ratio of film area.
In other words, you really need the one with the floating element.
This aligns with my experience, but clearly this is not universal. Plenty of people here and elsewhere love how sharp the 50mm for the RB is, while others find it unbearably soft. I wasn't able to print a sharp 8x10 from negatives made with the examples I tried.
So what gives? Is this just the difference between the Sekor C and the earlier, non-C versions? I am perplexed.