I paid $200 to get mine installed. The springs definitely stiffen up the ride. I find the "feel" of the car a bit degraded when riding in "comfort" mode. In normal mode the handling feels slightly different - I still can't tell if it's really improved. In sport mode, the handling feels noticeably better.
As for comfort, the only positive reviews I've read with aftermarket springs are with the Eibachs or Dinan ($$$)
2009 E92 M3 | Alpine White | Black Extended | Advan RS | Turner Test Pipes | Dinan Axle-Back | OETuning | Eibach Springs | UUC SSK | VRS Front Lip | VRS Type I Diffuser | Matte Black | RPi Scoops | MS Filter | Yokohama AD08 | F1 Pinnacle
Special Thanks: Gintani | OETuning | eAs