I was strongly considering buying a bike off of the site. All the research I did on them was that their bikes are sort of "Grocery Store Brand" bikes -- frames made in the same Taiwainese facilities as popular bike shop bikes, with a no-name branding (Motobecane and Windsor were bought out long ago), and the same Shimano parts you see on bike shop bikes. From what I gather, their pricing really is from volume sales and the fact that they outsource the final assembly to the customer. They do have one brick and mortar store in Florida. For high end Shimano and SRAM parts, the entire bike is comparable to the street price of those parts alone, it seems.
The downsides that I heard were that you don't support your friendly neighborhood bike shop, you obviously can't test the bikes so you need to the knowledge about what style of bike you want and the and size and geometry that fits you, and be able to read that from a diagram, and that you need to assemble them yourself (or pay $40-60 to have an LBS assemble it, and then pay to retune it a few months later once the components have broken in). Also the unmustachian concern that you have no brand recognition.
I ended up finding a used bike on Craigslist before I could put this research to the test, though.