Premium Economy is different things to different airlines. Some of them it can be simple things like the emergency exit seats. Some of them it's proper business/first class style section of the cabin with better seats, but how much better they are is entirely random really.
I've flown first, business, some sort of business first hybrid on American Airlines, and a few premium economy style seating arrangements too. Luckily I wasn't paying for them so I can't really comment on value for money, but as someone who is quite tall I can certainly say they were much appreciated, and that includes the most basic premium econonmy one that I flew.