Are they really up to 60% off? Has anyone ever bought a bike on the site?
I've bought 2 bikes from Bikesdirect, both hard-tail mtn bikes in your price range (like $600 for a motobecane, $800 for gravity). I've been quite satisfied with both, but minor issues rise up occasionally (like a bolt disappearing or front fork adjustment getting stuck). If you do any trails at all, you pretty much need to get a hard-tail mountain bike though you could get one of the more inexpensive cyclocross ones (I wouldn't).
As to whether they're really 60% off, they can be, but typically not. However, they're pretty much guaranteed to be a better deal than what you'll find at your local shop.
So, the real question is, how much effort are you willing to put into this? You can walk into local shop, have them check your fit, walk out with what's plenty good of a bike for your use. And if any issues come up, you know where to go. Or you could figure out what size you need, spend some time on BD, look around, compare the parts, take a minor gamble, and order from them. You'll get a better bike for a better price, but the fit may not be perfect. Or it may require some customization. So really, if you're willing to learn, go with BD. If you want the easy way out, go with LBS.