I think that this has come up before a few times. But I wanted to share my experience with the hm5 pads. I was at a local meet in Nashville where a member had the hm5 pads on his 650. These were the pleather version. I listened to them a bit and thought that they sounded pretty good. If I had been using my thinking cap, I'd gone over to my bag, pulled out my 650 with stock pads to a/b them.
Anyway, they were super soft and being pleather attracted no dust...bonus! They might have had a touch more bass presence as well. Then while surfing the classified here this week, I came across a new pair of hm5 pads. It happens that this pair was velour instead of pleather. But for the price I decided to give them a shot. I took an old pair of 650 pads and ripped out the retaining ring. It was not easy to get the new pads clipped back into place but it's doable with some effort and patience.
*memory foam is more comfortable than stock velour
*they are deeper than stock pads and also the inner diameter is a touch smaller than stock. Still very comfortable but a touch "snug."
*the sound is slightly different but the difference is not huge. The hm5 sound like they have a bit more air
*to get these truly comfortable they require an extra notch extension on each cup for me
*unless my mind is playing tricks on me, the hm5 pads require a extra notch on the pot to get to the volume level. Almost positive about this.
For the price, if nothing else they would be a good alternative if you didn't want to pony up the cash for the stock pads from Sennheiser. They sound very similar.