I'm currently in an airport right now, with a exit row seat assigned for my return flight home, and I know that the armrest will be solid like this (my earlier flight where I assigned myself a seat in the same row was like this).  While it sometimes sucks that the armrest is immovable (the middle seat was empty in my earlier flight, and it would have been nice to have been able to lift the armrest to spread out a bit), overall I'm so happy that they can't move-- even if I'm seated next to someone huge today, they won't be able to invade my space!