While certainly a notch below in terms of quality compared to the original earpads, these replacement ones are as good as they come. How do I know? I had ordered about earpads from Aliexpress while they still operated in India for about 400 rupees, and those were certainly worse than these ones.
The hard plastic ring is rigid, the underlying rubber is tough (the cheaper ones I bought from AliE already had holes on the inside when it shipped to me). The velour fabric is not as great as on the original ones.
I felt as if the sound became a bit echo-y, but just a bit. The seal could be better (however, these need more time to get moulded to my head's shape). If you were to compare the same headphones but with different earpads side by side, you will probably find the replacement earpads make the audio muddy. But once it's a few days, you won't remember what you used to hear and get used to the new sound.
Finally, I feel with Sennheiser not giving a flying fuck about after-sales service or parts' availability for these headphones, this is quite a good purchase. My headphones definitely look much more appealing now.