I purchased a complete MTB via Competitive Cyclist. They are now owned by Backcountry.com out of Utah, who also owns Real Cyclist and Huck-n-Roll (now folded into Competitive Cyclist). Their customer service was very helpful throughout the process, especially when discussing sizing/fit and when pricing different options. Overall, pricing was good (no tax, potential for free shipping), and I was able to build a bike that would have cost significantly more at a LBS. The bike arrived nearly fully assembled, and had been tuned in advance. I just had to slap on the bars, wheels, and pedals, then off to the trails. All manufacturer warranties still apply, and Backcountry stands behind their products and will help with problems if they occur.
The experience was nowhere near the personalization of buying a bike at a LBS, and working through your LBS is much more convenient if issues arise. Unfortunately, I know that I will be treated differently than if I bought my bike at the LBS, but hopefully we'll still get along considering I am still spending a bit of $$ on parts and service.