It seems the odds are very high that you will be stuck next to a terribly large person. I tried paying extra for the legroom and when I looked at the seatmap the day before the flight, I was the only one to choose this option on RDU-JFK. What I think happened is they took the 3 largest people at check in and placed them in the exit row. I can't think of any other explanation.

All I know is I had to lean over against the wall the entire flight and I couldn't touch my TV controls without asking him to raise his arm.