Buying bicycles online
It's a legit company. Many of the bikes they sell are not really name-brand bikes—they're house brands. As I understand it, Mercier and Windsor are old bike marques that went defunct, and BD bought the names and sell their bikes under those names. There is a certain misplaced snootiness about this, since almost all these name-brand companies outsource their manufacture to a few giant factories in China or Taiwan and slap their own decals on. BD's other bikes are remainders from other companies, I think.

Many of their bikes skimp on the specs, substituting lower-cost parts in places you won't notice (like the bottom bracket). Of course, lots of name-brand makers do the same.

Finally there's the assembly. I don't have direct experience with BD bikes, but I've read unkind things about the quality of assembly. Since you're buying a pig in a poke when you buy mail-order, you don't have a chance to see if the wheels are true and evenly tensioned, that sort of thing. In that respect, you really are getting something worse than you'd get from a reputable local bike shop.

If you know what you're getting when you order, they're fine. But you could be buying a bundle of problems caused by poor assembly. You're definitely not buying name-brand cachet. And might not be buying a bike with quite as good specs.