I think it is a great tool if you are prepared to replace it on a frequent basis. Otherwise, it's just an ideal location for all sorts of bacteria to grow as it is difficult to clean and disinfect.
Personally, I prefer brushes as I can clean them thoroughly, but the BB can be very versatile in situations where you don't have your whole arsenal of brushes at your disposal (e.g. holidays). It's a clever product, but pricey for what it is, especially in the frequency in which you should get a new one.