I have a few times. The last time was ~3 years ago when I few to Mexico for a friend's wedding. The cost between J and E- was like $120 which made it a no brainer. On my recent trip to Europe I did spend the extra 70 bucks to get an MCE seat which made the flight from ORD to LHR quite pleasant. We'll see how the exit row on the 380 is on my way home next week.