Honestly, I have no idea how they acheived their high emissions test pass repair record. Nonetheless, according to the Illinois EPA, Factory Muffler has the best emissions repair record in Chicago.
I've been there myself to get a check engine light diagnosed and repaired. I got that fixed and also replaced a muffler and a front axle.
I have no complaints about the quality of their work, but they don't offer as good a warranty as some other places do. In addition, I get the feeling their prices are somewhat negotiable. Still, they have a good track record where it counts.