Art Coop has two main things going for it: it's close to campus, and it carries pretty much any basic art supplies you might need. Otherwise it's pricey, and most of the employees I've talked to their haven't been very friendly.
If you need some art supplies right away and are stuck near campus, I'd say go there. Otherwise, if you do a little planning, you can just order your supplies online and save some money and hassle.