Their prices are the best in the area. They always have some kind of daily deals, and they had 10% off everything for the whole month of April (20% off on 4/20).
They give you $25 of in-store credit every time you spend $500... If that wasn't enough, they have the most extensive selection of carts I've ever seen. The staff is totally cool, very knowledgeable and accommodating. And, they have free bags of chips most of the time.
Besides all that, that they only cater to medical patients, so you know what to expect when you walk in the door.
They have a small parking lot, but I've never had a problem getting a spot. And hey, most places in Chicago don't have a lot at all (or free chips!)