I have a 2013 SX 7 passenger - we have a 05 Grand Caravan as the people mover so was originally looking at the 5 passenger. My oldest daughter (15) came shopping with me and after looking at both the 5 and 7 passenger we opted for the extra seating (in the event we did have company visiting), sometimes it can be a real pain having all 3 kids sitting side by side (second daughter is 12 an son is 10).
Both girls have sat in the rear seat together and have not complained about the visibility (they seemed more enthusiastic as they were away from their brother!!!) - but once my son sat there by himself and I could hardly see him in the mirror - he did have a hard time seeing anything out the side windows.
I am 6'2" tall and have tested the rear seats - yes they are a bit small and my knees were up to my nose, but I was sitting in "relative comfort".
Before we bought the Sorrento our dealer let us use if for a weekend so we were able to test it a few times before we signed the deal.
I would really call the 3rd row a "just in case" option but if you intend on using the seats all the time it may require some additional consideration and testing before you make the deal.
One other thing to consider with the 3rd row in the 2014's is the fact that Kia removed the "fold forward seat" - to gain access to the 3rd row in the 2014's you have to climb over the second row passenger seat - in the 2013 the seat back folds down and the bottom also folds forward giving you easier access to the seat - seemed a bit strange to me, but was one of the reasons we pick the 2013 rather than a 2014 (I had 3 salesman trying to fold the seat when I asked how to access the 3rd row before they realized the seat function had been changed).
Hope this helps!!!