I'll give a quick little review on what it does.
For one, the seal on these is just... ludicrous. Between the stiff Shure pads on the bottom and the very squishy, suction-y AKG pads on top, the level of noise attenuation that these provide is just insane. I tried a Bose QC15 once, and I would go as far as to say that this gets very, very close to that level of noise attenuation. Weirdly, though, they seem to do more isolation of bass and mids than they do of highs, which in my experience is the exact opposite of how most headphones isolate.
I first tried to do this with two Shure pads, and the clamp on the cans was just unbearable, and I felt that the headband assembly was going to snap in two every time I put them on. Once I switched to Shure-on-AKG (and effectively destroying both pads for normal usage at the same time), the clamp became very comfortable and the very light density of the AKG foam became a positive.
The bass is vastly improved when this mod is performed. Since these seal so effectively, the bass just pops up in the mix, and is very well-controlled and extends deeper than normal.