ya.sigma service is pretty good.
my experience was the tech will take some pics using the focusin chart and show u the before/after results.
my 1st visit there, didn't make appointment n went earlier than their opening time.
the auntie recep was kind enough to allow me in and even take in my lens on behalf of the technican.
For everyones sake, pls try to confirm the focusing before going for a calibration. Yes, its free, but if everybody does it when there is no issue, its just going to take up the response time. End of the day, all of us will suffer extended turn around times. Even worse, past warranty lenses may be charged.