I've had the clutch masters FX400 stage 4 w/ lightweight flywheel in for over 2500 miles and I like it.
I previously had the DXD Southbend stage 3 endurance w/ lightweight flywheel, and the pedal was around 10% stiffer than my FX400, which really surprised me as its a higher level clutch.
Regarding chatter, my DXD clutch was much louder. With the FX400, I do hear chatter. If I pull into my driveway and put it in neutral, you hear the chatter. Engage clutch, shift into 2nd then 1st then back to neutral and the chatter gets significantly quieter.
Super loud/horribly uncomfortable to drive for the first 750 CITY miles (stop n go miles, roads with lights, etc. Highway miles you cannot count as break in unless you are changing gears.) After that, it was like a different clutch. Broke in very nicely.
Overall, I really like it compared to my DXD stage 3 Endurance.
The FX400 puts 360whp to the ground very quickly. This thing GRABS like a mo'fo, and never gets sloppy.
Thanks Clutchmasters and R.A.I. Motorsports.