I resisted buying this brush cleanser for a long time. But as time went on, and my MAC brush collection grew, I broke down and got it. And now I can't live without it.
I have heard people complain about this brush cleanser, because it is too expensive and it is drying on the brushes. My last bottle of brush cleanser cost me about $15, and lasted me several months of DAILY use. I consider that a pretty good deal for ensuring that my brushes are well maintained.
I have also not experienced problems with the cleanser being drying. I do condition my brushes (with hair conditioner) when I deep clean them. All of my brushes are as good as the day I bought them, despite the fact that some of them are over 5 years old.
I spot clean my brushes everytime I use them - and a brush cleanser is the way to do it. I will post a tutorial on cleaning your brushes in the next couple of days, so keep an eye out for it.