It should be pretty accurate and Truecar is a legit operation. If anything, the prices may be on the high side. Truecar changed their model recently. You used to be able to log in and get a commitment free no haggle price from a participating dealer. They ran into some legal issues and griping from some dealers and manufacturers, so now it's not much different than other sites as far as I can tell. If you're interested, this is a pretty good read...
After A Near-Death Experience, A Kinder, Gentler TrueCar Is Re-Born | The Truth About Cars .
It's important to note that "dealer invoice" is almost never going to be what the dealer actually pays for the car. Manufacturers provide incentives that are not included in the invoice price that you see. For example, many have what's called "conquest cash". If you're driving an Acura, for example, a Lexus dealer might give you $1000 off. Most of the time you don't even have to trade in that vehicle. There are also incentives given to the dealer to sell new or slower selling cars. The manufacturer might give them $1500 for each specific model sold. The dealer won't disclose this to you though. It gives them more room to bargain as they may "give you" $500 of it and keep the remaining $1000. There's also financing incentives that are even more behind the scenes. It's all part of the game... Edmunds lists current incentives on their website:
Incentives and Rebates .
Good luck shopping. Hope this helps!