Terrible! Went in to have a sensor changed and, being that I drive a high performance vehicle and do my own maintenance when the weather permits, I naturally purchase my own parts. Upon arrival at Simon's (because they "don't give quotes over the phone") I was told by the disinterested gentleman behind the counter that they also "don't install customer parts" (pretty long list of things they -don't- do, huh?). Obviously their mark-up on parts is so significant that they don't want your business if they can't hit you with an insane part cost in addition to labor! What a racket!
Even though it's on the other side of town I'll continue to take my cars to the Detroit Auto Clinic: they don't care if I buy my own parts, they're just happy to have my business and do excellent work.