I am telling you if I had the money I would do it every single time.
I hate every single minute of flying in the shitty normal seats, any improvement I would pay it.
Well I already do for the extra space, first rows seats and so on....I haven't but sometimes I have been tempted to buy the seat next to me as well.... but it would require to put somebody else details I guess and they probably with my luck simply fill the seat anyway if the passenger doesn't appear.