I got an N4 from the Rezound, great upgrade... the N5 is even more robust, battery life is phenomenal compared to the Rezound, which was pretty bad actually, you wouldn't be disappointed unless you try to use it on Verizon, it won't work. As far as "functionality" goes, it is plain vanilla Android, so if you are used to Sense and some of it's embedded features you might be disappointed out of the box, but there are tons of apps that can do almost anything, you might just have to do more tweaking depending on your use.
Is it worth upgrading, yes... the difference is extreme. Should you wait, that depends on you... there is always something better right around the corner, so even if you wait, when the Nexus 6, or Moto R, or whatever is announced, there will always be something else better coming out soon.