madVR and Darbee do different things. I have a Radiance XS and a Darbee in my output chain. The Darbee enhances visual depth cues by selectively sharpening and adding contrast and realism to an image.
I use both. The Radiance due to its excellent scaling and deinterlacing. It also provides for automated display calibration with either Cromapure, Spectral or Light Illusion software. It also is a great HDMI switching hub. I send 1080i60 video to the Lumagen XS and let it deinterlace my HDMI from the HTPC.
My video chain is HTPC with intel integrated video>,AVR> Lumagen> Darbee> and finally the Mitsubishi 92" Rear Projector. The color correction of the Lumagen and the Darbee processor make the biggest improvement in picture quality.
Later Lumagen Radiance boxes than mine have Darbee integrated. The madVR renderer would further improve the video quality. I don't see the Darbee or madVR as a either or by any means. I don't use madVR much as I have chosen for convenience and usability reasons to stay mostly in Window Media Center. I watch more TV than film. I can tell you that if I had to choose between the two, hypothetically, I would choose Darbee. But again, if you can use madVR effectively Darbee will only make things better. Sometimes the 3D effect from the Darbee is uncannily realistic.
Samsung 78" JS8600, Mits WD-92840 43" Samsung 43ku7000 - Lumagen Radiance XS, Martin Logan Motion 40s, Ascend Sierra 2s
58" Panny Plasma -DVDO DUO, Usher S520, JBL HLS-610, Definitive SM45, MusicHall Marimba