I don't think it is a foregone conclusion as is so often suggested. Should Karjakin get a lead, even if it is only one game, then the match could get pretty dull as it would be tempting to try and do a "Kramnik" and just draw all of the games. Karjakin is an excellent defender, should he manage to take a game or two then an upset isn't completely off the cards.
It all depends on the score at the half way mark. If it is tied (or even a slight advantage to Karjakin) then anything can happen.
Having said that, i would be very surprised if Carlsen didn't win. Of all of the players in the top ten, I think Caruana would stand the best chance against Carlsen. Giri has proven himself to be very solid and would be a really hard player to beat, but i don't think he currently has the strength necessary to actually win the match. Aronian would be a good contest also, but i fear he has missed his chance to play a WC match now. He has certainly missed the opportunity to play one at his peak any road..
One thing to bear in mind about Karjakin is that he has never had the support that Carlsen has had over the years. That has changed recently though. He is an outspoken supporter of Putin, which given Putins active interest in chess could mean that he is getting a lot more support for this match than he otherwise would have.
When Fischer played for the World title he often complained about the Russians pooling their resources in order to beat him. Well, Karjakin comes from one of the strongest chess Nations on Earth! It is not unreasonable to suggest that he could gain the benefit of many dozens of seconds, training partners, not to mention finance. The Karjakin that sits down across the board from MC could be a very well prepared candidate indeed!