I've got a WER damper on a KTM 360sx and a scot on an 02 TM250. My wife has a scot on a 99 KDX200.
Riding rocks and roots they help a lot, less deflections so less crashing, riding sand they help over whoops (we get a few over here). The only bad thing is that since you can get away with hitting things harder you do, so when it does go all pear shaped you crash going faster. I think they are worth the effort. It only has to keep you out of hospital once to pay for itself.