So I thought I'd throw my two cents in...
Ordered and received a Darblet from AVS (a version with the Lumagen fix)... On a whim (since I intend to use it with our projector) I first tried it with our upstairs setup:
1) (source devices) -> Anthem AVM 50 -> Darblet -> Pioneer Pro-101FD (plasma)
This basically was a no-go... constant purple flashes (every few seconds) or blackouts, audio dropouts, the sporadic "HDMI compliant device not connected" message from Tivo. Didn't matter what source device... they all exhibited issues making the system unusable with the Dablet in the chain. So I was unable to evaluate with this smaller display device but no matter since the Darblet wasn't intended for this setup.
2) (source devices) -> Lumagen XS -> Darblet -> JVC RS-20 (projector) (NOTE: Audio is processed by an Anthem D2 from the second hdmi output on the Lumagen)
This was much more successful... the Darblet didn't seem to introduce any handshake issues (except for some purple flashes when changing channels on a Tivo that also involved a resolution change... but only during the change... once the channel displayed everything was stable). I have 4:2:2 configured for colorspace for all devices except the Tivo (4:4:4)... best I can tell the JVC was receiving the 4:2:2 colorspace as configured in the Lumagen.
I found that anywhere between 40-55% using HD mode on the Darblet was the sweet spot between "Not seeing much difference" and "Seeing undesirable effects" when switching the Darblet on/off... In that range the effect was noticable but subtle... but not the night/day difference I was kind of expecting based on some of the comments in this thread from some users (i.e. "veil lifted", etc.). I see a sharpening and contrast enhancing effect, particularly on faces, clothing texture, etc...
While I notice the difference utilizing the demo mode not sure that I can really appreciate the differences watching the same scene played though once with the Darblet engaged and played again without. Will live with for a while to see if it is worth the extra outlet and IR emitter