I just got a Filtrone and I love it. It sounds beautiful.
6/18 is the best setting since it allows crossfading between a fairly closed slope and a very open one.
I just wish there was a CV input for tone control (maybe instead of v/oct, if room was an issue), but besides that, I'm very happy with my purchase. Even though I have many filters already, this one is a unique addition - something I missed until it existed.