Well, lvl 5 BB does 16 more dmg over time than lvl 1 BB, if you think about it, it's not a big gain, but it's cooldown lowers from 15 sec to 10 sec. Personally, I wouldn't mind maxing it because I don't like GS and placing more points on 6th sense is a waste, but as I said, if you really wanna have GS with you then having just 1 point in BB would make it a nice build.