I've messed with these three:
(L to R - no name $30 knockoff JP Cooley, $35 ebay wannabe "dynacomp" , $85 Spike's Dynacomp Extreme)
#1 on the left works pretty well for $30. It's currently on the 308. Sideblast is "noticeable" but not too annoying and I'm getting improved groups over the standard birdcage A2 flash hider.
#2 in the middle works very well - used it on the 300 BLK and was a pleasure to shoot, but was a hair too short (14.5" barrel)
#3 on the right - impressed with #2, I decided to go with the Spike's Dynacomp Extreme on the 300 BLK so I could pin it on properly to make everything legal. I figure the performance will be about the same as #2, if not better. Haven't tested it yet.