I've only personally kept one; she was the dwarf species, which was cool since she maxed out at like 24".
I see the viper boas every once in a while in shops and shows, and occasionally a Bibrons. I think they're probably the most popular of the genus because they look so much like a venomous snake (very close to a cottonmouth to the layman).
Like I said before, its hard to compare their coloration to that of a carpet, GTP or ETB, or even an ATB...but I will say they're just as easy as caring for a carpet or ATB without the latter's snappy rep.