This. I was also one of those thinking 256 is plenty when I went from a 512gb MBA (2013) to a 256gb (2014) one. Found myself constantly compromising on space, so decided to go back to 512gb and is what I'd recommend anyone.512 for me, 256 is too much of a balancing act in the field. Once back home I can prune the garbage from the 512 and not be concerned with space usage on the job.
Me too - was so full it was getting painfully slow!This is my only machine. It feels GREAT to have 512 when I have been stuck on 120gb for the last 3 years!!!