Several years ago, I needed front rotors for my Yukon. I checked with the GMC dealer and they wanted $365/each.
I went down the street to AZ and they wanted $65/ea. Well, that astounded me and I was thinking, "Sheesh, they must be crapola at that price." So, I called my brother who worked for a company which makes rotors for Detroit and asked him the difference bewteen the ZA rotors and the OEM rotors.
His answer? The paint. Apparently, the OEM rotors are electrostatically painted and the AZ rotors aren't so fancy.
Needless to say, I went with the AZ rotors and haven't had a problem.
I've also had the opportunity to use an AZ rebuilt alternator and haven't had a problem with that item either.