I have low-B 5's, fretted and fretless, because they're necessary. If a guitarist doesn't like it, they can just go back to the 20th Century where they belong. I don't have a 6 because there's no reason to go up. Having said that, I do have one of these just for the shear ludicrousness of it…I bought my first six-string bass this summer after playing four-string basses exclusively for over 15 years. Honestly, I can't imagine going back to four (Ok, I could probably do with a five). Having the extended range of the B and, although to a lower degree, the C strings without losing what makes a four a four seems like a no brainer.
Why haven't you made the change to a six or even five string bass already?
…essentially a long scale old school 6-string bass with the added B.