I purchased the Dekoni velour pads for my HD650s which are 9 years old - I had worn the originals down to almost nothing. The Dekonis were cheaper than buying official Sennheiser replacements and fit very well. They are firmer than the Sennheiser pads that come with the HD650 and HD6XX (I have a pair of 6XXs as well) and the cups are deeper which is great if you find your ears touch the inner foam a lot with the stock pads.
In terms of sound, there is a noticeable increase in warmth and bass with the Dekoni pads. Personally I prefer detail and resolution over bass and warmth, but I may be able to EQ the headphones to deal with this.
It would have been nice if they had been able to include new foam discs with the pads, as my discs are also worn out and full of holes, but I'll just source some elsewhere.