I have an LTR in .223 that's got a YHM QD flashider on the end. The accuracy drops a little with the can on, but not too bad. On my .308, the groups shrunk in half (at distances of 300yds or so) when I took off the YHM QD 762 and replaced it with a different brand (Griffin Armament Scout) thread-on 762 can.
My 556 QD can is still pretty tight against the retention ring, but the 762 can wasn't. The little spring washers in my 762 QD flashiders must be getting worn out or something. I need to have YHM send me some replacements I guess.
I still think the YHM are a good value, but I sometimes wish I'd have gone with an AAC M4-2k... Oh well, the YHM QD 556 is GREAT on my 10.5" upper; perfect for PR matches and general varmint control around the house.
Here's the LTR w/YHM: