I tried mine two weeks ago with a variety of shells to get a feel for which spring to use. There is a significant recoil reduction with all of the high brass loads; however, with low brass, I don't know if the third spring is even engaged (Including your stock spring, there are three springs in the recoil reduction system). I shot a three gun match on Sunday and used the gun with the recoil reduction system. The gun ran flawlessly, no issues with chamberring.
I only use my M4 for competition and I only run low brass ammo. The recoil reduction is not significant with low brass. Again, you feel it with high brass shells.
I will put mine up for sale. Ordering from Zahal, it will cost you $134 ($150 and 12% off/free shipping offer) and take 2 months for delivery from Israel (the hold up is through Turkey).
You can have mine for $120, shipped.