I've been buying their diesel at the South Jordan, UT location for quite some time now. I have no complaints, except that I'm sure they're doing a bio diesel mixture. I can't find any documentation about it, nor is it labelled on the pump. Without a shadow of a doubt, there must be at least a small amount because it changes the exhaust smell the same way running B100 does.
Not really a complaint, I guess, but I won't be putting it through my 2015 until I can get some clear answers on the percentage. As far as prices, they're consistently the lowest in the area.
Edit:
I just spoke with a representative at Costco. Sounds like they plan to add diesel to "as many fuel stations as possible". I wish they would sell fuel after hours now.