I finally got a chance to try the XLR, on a flight to Zurich.

The originally scheduled plane was a regular A321. One day before the flight, I get a message from Iberia saying that the plane had changed and they had assigned me a new seat with the same characteristics. I had, like always, an emergency-exit seat and my new one was in row 28. This meant XLR, so I opened my booking and was immediately offered an upgrade to business class for € 109. I declined, but I was tempted to place the lowest possible SeatBoost bid (the first time I have used this system), for € 47. The upgrade cleared and I got to satisfy my curiosity.