I use Eco earth. You can't beat $5 a brick. It looks nice and holds humidity well. The only thing is in my experience if you leave it wet (not damp, more like soaked) then it can mold. But this is the case with most substrates I believe.
My BP pushes it around pretty easily though, which can be annoying when it all looks uneven if you care about that.
It lasts a long time, and is easy to spot clean. I want to try Repti bark some time, but Eco earth works pretty well and is inexpensive.
Edit: I just want to clarify that I don't nor have I ever owned boas. I only have my BP, but this is my experience with Eco earth.
I use a mix of orchid bark and eco earth as i find it a bit better than just the eco earth, the only annoying things it gets everywhere, especially in the water bowl and when I pick my snakes out god knows how they seem to grab a coils worth and drag it all over the side , forever hoovering the runners out and sides off, don't have your food to wet either or it really sticks especially if it's a bit dry on top,
It's nice and easy to spot check when cleaning, have to re wet it now and then to stop the dust too, mine like to burrow in it if it's deep enough too