I find WTP/premium economy is very worthwhile as economy cabins continue to be densified. The extra width makes a material difference for me. But the key is to book directly into WTP, preferably during a sale or as part of a package, rather than booking WT and then hoping for a POUG or AUP. I'm sure it varies a lot by route/season, but for the routes I travel the price difference between WT and WTP at booking can often be lower than the POUG price I've been offered the few times I booked WT and then regretted it later.