The first half of Final Fantasy XV will be open world, and the second half linear. Kotaku reports game director Hajime Tabata explained the games layout in a recent interview with Japanese magazine Famitsu...

“As for Final Fantasy XV’s structure, it has both an open world part and a linear part. The open world [element] continues throughout the first half, and because of the story, the second half progresses linearly...If you do it like that, then you won’t get tired of the open world, and as the whole game becomes more focused, you can enjoy a traditionally Final Fantasy style progression."

From the demos I've played, I kind of expected a lot of linear gameplay. For example, the Colossus battle that we showed you at E3 felt a lot like the Platinum Demo that was made public a while back. It's still very fun mind you, but when we see mostly see footage of guy's driving a car and roaming through the wilderness you're lead to believe it's mostly that. 

Tabata also added that it will take around 40-50 hours to make it through the main story arc, which is to be expected, but still welcoming to hear. 

From what I've played, the game feels very much like Kingdom Hearts. If you're into that (I certainly am) then you're going to like this game. If you're looking for a more traditional FF experience, I would suggest looking to one of the remasters.