OK, I just installed an Advanced Armament Corp cyclic rate reducer into my pre-ban M4A3 and I can actually tell a very noticeable difference between shooting the gun with the standard spring and buffer and the AAC spring and buffer, especially in rapid fire.  

Has anyone tried any other recoil reduction systems that have worked well?

I bought another AAC cyclic rate reducer for my Bushmaster Varminter but I have not tried it out yet since I just got the gun and have the scope setting loose in the rings waiting to get time to level the scope and sight it in.  I only had 30 minutes today to test out the AAC reducer for the M4A3 and two Harrts recoil reducers, one for my Sig Sauer P239 .357Sig and one for my P229 .40S&W.