Of course this is going to piss them off. You're shrinking their margins and taking away money from them. Let's see what happens when a person goes to a dealership and says, "Give me this Honda Pilot for $x because you gave it to so and so for that much".
Not a dealer. I'm just seeing how much pull a site like DealerDrop.com has when the user actually tries to negotiate a deal. I think if the customer has discipline, they can use this site to get the car they want at the right price. They just need to make sure they don't fall into Dealership tactics of upselling their product (i.e. extended warranty, GPS system, upgraded wheels, etc.). Whatever the dealerships do not make up front, they'll get it at the backend.