I had two occurrences where I needed Sigma service. Both were on my 70-200. The first one was some sort of dirt which had accumulated behind the front element. I would say it was some sort of fungi since it was a prefect ring forming at about . Sigma cleaned the lens for free but added the disclaimer that this was not a warranty issue and that the free service was voluntary from their side. This time it was send through the store and I had my lens back in about 5 working days.
Second occurrence was a broken AF at the outer edge of my warranty (Three weeks later or so and I would have had to find out just how flexible they really are). I thought to be clever and actually brought it in myself since their German service station is rather close to my work. turns out that this was maybe not such a good idea. The lens was repaired for free, but this time it took them something like 3-4 weeks. Though to be fair their service station is not really equipped to receive equipment for repair, so it might have gotten lost in the shuffle a bit there.
Both repairs were flawless and both times the lens was very clean and free of any visible dust inside. (Which was not the case before the repair, though it never affected image quality as far I as could tell) The lens held up after that, at least until it was stolen
This was Sigma Germany btw...