Except in my experience, Carwoo doesn't work (and just ends up like what incision said for #1 above).
Interesting - I wonder if it's your location. I used it a few months ago (I'm in NJ) and it worked great. Price was way under TrueCar, and going into the dealer was horrible. Only downside was I had to drive an hour away to get the car but was in and out in 30 minutes.