I've used one before for a brief period of time, and I think the newer Dual Bass Wah (PBA-2) sounds better overall. I honestly don't remember what the old blue one sounds like anymore, but I do remember it working well on bass (as it should

).
One of the TBers who used to moderate this forum - tplyons - had one and swore by it. He's still around - I'd send him a PM and see what he thinks.