I have been yes, I run 60rnd mids.
So far I've noticed they would always struggle with the last few in the mag, but sometimes would just say no half-way through a mag. Similarly, I've noticed that the first few shots out of a full mag aren't full power..
I've read from various places that new mags need breaking in. Not sure how much truth there is to that, but may be the case as I remember having feeding issues with ASG Evo mags at first. In my opinion the springs do feel quite stiff for both the main spring to feed the BBs, and the spring for the stopper lever.
Only used for 2 skirmishes so far, so hard to say at the moment if time will resolve..
(I use G&G 0.28 BBs)