On an exchange (like Betfair), you are betting against other people. Supply and demand sets the odds. On a sportsbook (like Bovada), you are betting against the sportsbook and their employees set the odds. When you hear things like 'the odds are designed to get people to bet the most money' they're referring to sportsbooks (even though they may not realize it). That's not how exchanges work. Exchange odds are a reflection of what the market thinks the true odds are.
Sportsbooks make money by adding 'juice' to their odds. Say the Bears are 100:1 to win the Super Bowl. They'll offer their customers 80:1, the 20:1 difference being their profit. Exchanges have no juice, so if the market is highly liquid, you'll get 100:1. Since the odds are 100:1, the exchange is more accurate than the sportsbook.
Betfair is by far the largest and most accurate exchange. Betfair is so accurate that what typically happens is sportsbooks simply mirror Betfair odds and add juice -- literally taking Betfair odds and making them less accurate.
*Betting on an exchange isn't necessarily a better deal. Exchanges charge fees and sportsbooks are generous with bonuses. It can also be hard to make large bets on an exchange withing changing the odds significantly.