I picked up a fulla a few weeks ago for my HD598's and for mobile laptop use with my RHA 750i's. My biggest qualm is that with my HD598's the audio can randomly get glitchy when the cable isn't in the perfect position. Also, turning the volume knob can cause a slight static sound, which isn't bad as the sound only happens while the knob is being turned, and I usually just keep it at one setting and change the volume on my actual PC.
Besides that it was a worthy pickup(for laptop use especially, as for some reason the sound output from my laptop is pure **** on its own.) It helps fill out the HD598's sound(especially on the lower end), so in my opinion it's a good purchase for even a pair of easy to drive headphones like those. I have a very old pair of VMODA LP2's and the sound quality hardly changes with those, but that's to be expected.
I'm going to be looking to pick up some HD650's,X1's/X2's,or HE400's/400i's in the future. Has anyone had experience with the fulla+one of those? Namely the HD650's.