Update 6 June 2010
I'm still dialing in my rear suspension on my 09 SXT I. The current coil is a little weak (>35% sag) and I just got a 50lbs heavier spring from my buddy, but haven't installed yet to check it out.
However, just curious what other riders w/ this frame and a appropriately sprung DHX 4.0 coil (or DHX 5.0 coil) are using for their settings (Rebound, ProPedal, Boost Valve).
Undersprung, I was running:
~195-200 lbs rider weight (~175lbs rider + 20-25lbs pack/armor/gear/H2O/etc.)
-600# Fox steel coil
-5 clicks R in from full fast
-6 clicks PP in from full open
-175psi Boost Valve
-200lbs total rider weight (rider+gear)
-650# Fox steel coil (0 preload, just the std 1 turn after contact)
-10 clicks R in from full fast
-0 clicks ProPedal
-130psi Boost Reservoir
The bike has seen about 50% shuttle/ 50% earn-your-turns (fire road climbs). I tried the 650# coil in January, but it felt too firm. That might have been complicated as I also tried a firm spring for the Domain (318 MoCo) the same day. Anyway, I ran the 600# coil pretty much all winter, with a fair amount of sag. Compression was about 2-4 clicks on the Domain, with anywhere between 5-8 clicks ProPedal on the shock (depending on whether it's more fast downhill or more drop/stunt filled) and about 175-200psi in the reservoir.
Last few rides it really seemed like the rear suspension was too soft. Yesterday I ran the current setup and the 650# seemed like it worked much better than previously. Down a fast choppy section, the suspension felt glued and tracked really well. Pedaling was much more efficient, which translated into a snappier bike on the trail. I really only felt a bottom out once, and that was a fast g-out drainage section. Ran 5 out of 10 clicks (more than normal) on the fork to firm up and balance the suspension out, which worked great. Surprisingly, the fork seemed to track just as well as when it was running less compression damping. Overall the Domain has been a smooth, solid fork. Fluids have been changed once, but nothing else done besides a stanchion wipe down post ride.
It's a great feeling when you finally start to find that sweet spot-- completely changed my confidence and riding yesterday. Hope this is helpful to someone.