This site was recommended to me a few years ago by a friend of mine who works for a large architectural firm that builds all its computers in-house from parts bought from this site. Since then, I’ve built half-a-dozen computers and have bought parts for several others from them, without any problems. They are almost always cheaper than anyone else, with the exception of extraordinarily large or small items which run more expensive due to shipping costs.
A couple years ago, a motherboard I got from them quit working. I used their on-line return form, and within an hour I received an email stating that they no longer carried the same part, and offered to replace it with a later model motherboard from the same manufacturer, or pay me back the purchase price. I chose the former, and had my replacement within the week.
One of the unique features of this site is the review system: unlike most .com stores, a large percentage of the products sold have multiple reviews by customers, and if an item is consistently rated poorly, it is removed from sale.
The site’s organization also stands out: items can be separated by features, brand, and price in any combination, using either the “power search,” enabling a wide variety of options at once, or by “drilling down” through several searches, isolating items by category.