Checking out truecar.com where I can shop for the best deal on a car across many dealerships...

[wyldwolf]

... it's like a car buying exchange. Wait, what?? A car buying exchange?? How dare they! It's socialism! It's... Obamarides!

[n2doc]

1. It's ok as a starting point

You can do better in most cases with the traditional haggle. But it does give you an idea of where to start. And to weed out the pretenders in the dealerships.

[upaloopa]

2. If you are borrowing to pay you need to check out interest

rates also. Your payment is based on rate and length of loan.
And if a trade in is involved is the amount owed on that if any less or more than what you can get. Check Kelly Blue book for trade in value.
Then there are dealer incentives. If you get the inventory of the dealer you like look up the VIN number and you can get what the dealer paid for the car and the sticker on the window.
Another good thing to do is get pre-qualified for a loan.
Walk into the dealer knowing everything there is to know and name your price.
Basically they all sell the car at the same price based on zip code but the have reasons to reduce the price based on incentives and their sales for the period etc.

They pay floor plan interest. The longer a car is on the lot the less of a profit window there is. That was the way it was maybe changed by now.

[Daemonaquila]

3. I'd call it useless from the get-go.

You can find the theoretically best price at any number of dealerships, but the key for me is whether it's an honest dealership or a bunch of scam artists. If that best price shows up at a dealership I wouldn't walk into if my life depended on it, I wouldn't go there. When I go car shopping, I go shopping for the people I want to buy from... and I'll trust my haggling skills from there. About 50% of dealerships at any given moment will have the car I want at close to the same price.