I've used them on my 4 string religiously for 20+ years. When I bought my Lakland 5 several years ago, I was bummed that the RS66LD wasn't long enough. I even emailed Jason Howe and he just told me to buy the RS66EL (extra long) and buy the "B" separately. I've since found they sell them in a 5 string set now (RS665EL), and have been using them for over a year.
To actually answer your question: I think the low B is fantastic. Tight, focused, and bright. (FWIW I'm playing a 35" scale, so that probably figures into it)