Word from a couple of my FN cronies is that the out of box accuracy is superb. Much better than the FNS which to me wasn't to shabby.
I too really felt the mag release is MUCH improved
As already stated, if they had better aftermarket and cheaper mags, I would most likely go with FN almost exclusively. I have always liked their products. Hopefully they will work out the aftermarket kinks with the 509.