Airlines are very adept at charging us more for what used to be standard. Remember the days when you could simply just select any seat in coach that was available? Alas, those days are long gone and we are in the game of offering "enhanced" economy products. Enhanced could mean as little as an inch or two of extra seat pitch or as much as a bigger seat, better meal, more drinks and a seamless airport experience. Every airline has a different name for their product and the services and or values change greatly depending on the route and duration of the flight.
Although each airline offers different amenities my general rule of thumb is the longer the flight the more worth it. I would hardly ever recommend upgrading to any sort of "premium economy" on any short haul flight. Smaller planes, often used on shorter routes really only offer a couple extra inches of pitch in the front of the cabin and give you an eye mask. Hardly worth the $50 plus dollars each way. When the proposition changes to an overnight or transcontinental flight, that extra money can be worth it's weight in gold....
Long haul flights are an entirely different proposition. If you are flying on an intercontinental flight there is a significant chance that Premium Economy may be an entirely different cabin than economy. Premium Economy on Virgin Atlantic, British Airways, Air France and others offer a separate cabin, separate check in, better meal, welcome glass of champagne and other amenities which can really boost a trip. Is it worth it? This is simply a function of price, during peak times when a standard economy seat can run $1000 dollars you might as well spend $200 more to grab some great benefits and a comfier seat. If you are looking at great discounts on basic economy and premium economy is double, it's probably not worth it.
A great trick which is especially useful on undersold flights is to wait. Airlines are always looking to up sell and though premium economy may have cost many more hundred of dollars at booking, they may be willing to sell it to you at the airport for a fraction of that cost. It never hurts to inquire about any upgrade opportunities at check in. Sometimes you can even use miles!
Has anyone flown in Premium Economy before? firstname.lastname@example.org