I know that this brush cleaning soap isn't hugely expensive, but soap scum can be removed easily with stuff that you already own. A really light palm lather with either dish soap or shampoo has worked for me, followed by a diluted rinse with vinegar and water at a 1:4 ratio. Rinse thoroughly, and let it dry. If any vinegar smell lingers, rinse it again with water and lather with something pungent like Arko.
Just thought I'd chuck in my two cents for the budget minded. If you rinse your brushes thoroughly and have a couple to rotate, I really don't think soap build up is going to be too much of a problem.
For the sake of debate, I'll pose a question. Do you think that the softening of tips after using the Da Vinci soap is a result of the soap itself, or just additional use breaking in the bristles?