I've been using it for some time now. I'd personally call it "The Shave Killer"! Before it became available I used Shave and the Haircut (as it was the only serious hair solution for C4D available back then). Working with hair was painful! Now, with the Hiar module, it is actually fun!
It is 100% integrated into the program (no need for strange, "bridge" editor), the number of options and functions largery outnumbers those in S&H by Alter, seems to be very stable and what is most important, it is incredibly fast (both preview/work and the rendering), compared to other similar solutions. Definitely worth the price - if you're already using S&H I can only suggest to get rid of it and get the Hair module.