I bought one of my Macs from them. The reason? While they tend to stay pretty close to stock Apple pricing, they often add value with bonus items. For example: extra ram, free stuff (printers being most common, carrying case, accessories), etc. Extra ram is always a nice bonus. And free stuff can be sold giving you a sale price discount without getting the discount at checkout. When they are doing lots of bonuses for the same price as Apple with no such bonuses, that's added value for the same money.
Right now, I see they are offering Parallels 6, and HP all in one, and a "grab bag" promotion of little cheap accessories, plus a slight discount off stock prices. Or buy the same product from Apple and then pay extra for Parallels 6, an HP all in one, and any grab bag promotional item/junk. Don't care about those extras? P6 might sell for up to 39 (the lowest I've seen it offered new). The HP might sell for $20-$100. Etc.