I bought my dad's 2002 Toyota Tacoma X-tra Cab and my sister's Toyota Rav-4 L from CarsDirect. Since both situations involved cash sales (parents refi'd the house, sister got a settlement from work), it was a piece of cake. Got a way better deal than any dealership, and they delivered both cars to the house on flat-bed trucks.
The only issue is that, like any dealer, they primarily sell cars that are already on a lot somewhere. They have a large pool of local dealers, but if none of the dealers have your exact model, you either have to order it (which they discourage, due to the time involved), or take the closest thing they have, which is usually pretty close.
In my example, I wanted bucket seats for my dad's Tacoma, but that isn't a standard option for his model, and no dealership had one. But the "bench" seat was really two buckets with the driver's seat having a small extension, so it actually wasn't a big deal.
I'd do it again.
-Troy