I used them three years ago and it was a great experience. I'm not sure if the process is the same, but I configured my car online and a carsdirect rep called me within a few hours letting me know that they would be searching the dealers in my area for my exact configuration. A couple of hours later, they called me back letting me know that if I wanted my Xterra in black, that it might take a week. I told them it was no problem since I wasn't in a hurry. They called me two days later letting me know that my SUV was available at a local dealer for pickup. When I went in, most of the paperwork was already done and I probably spent about an hour or two at the dealership (mostly talking about carsdirect with the sales lady)
I did use a carsdirect.com price to negotiate a minivan for my mom at a local dealership. While I spent three and a half hours there, I still got the price I walked originally asked for.
Like I said, I haven't orderer through carsdirect.com in a few years, but I assume most of their process is similar.
Sorry so long, but I hope it helps.