Not sure who else uses it, i only know Meyson Jewellery in singapore uses SIG and only mainly to grade their Hearts and Arrow.
Meyson's price is pretty decent, pretty much the cheapest B&M chain you can find in singapore.
As to how their grading goes, i tried to call them early in the year to ask how much it would cost to grade a stone, but they said their grader had just left, so i don't think they're a very big outfit.
From my limited experience of living in Asia and seeing only 3 diamonds graded by them I would group them with the EGL international level. I wouldn't worry too much about color, but clarity might be an issue. The 3 I saw were SI1/SI2, but they have little carbon spots that you could see with the naked eye from about 3 inches away. Not obvious. I don't think the owners even noticed them.
I was under the impression they were a tiny lab. I know several people took the SIG grading to be more of an appraisal vs. lab report. Hope that helps.
Looked at some SIG diamonds under the loupe before. I would say the clarity grades are pretty accurate compared to GIA for SI diamonds. Can't use this as a basis to generalize their grading though. Color wise, mounted rings that I had seen don't give a fair assessment.