Yes, I flew Delta Comfort + to Rome last year and it is probably not worth the cost unless you need the extra legroom. On the way out I was in the end seat of the middle 2x3x2 layout and except for the 4" of extra legroom, everything else is identical to economy class. On the way home I was in seat 15E which was the first row of comfort plus. At 6'3" I could not stretch my legs out without hitting the bulkhead, but I could stand up in front of my seat without bothering anybody else. In general, I wouldn't fly Delta to Europe again if it means boarding a 767. The planes are old so they have small overhead bins which makes boarding a slow process. And the video monitors are small too.
Premium economy is much different. On a Lufthansa 747-800, the seat pitch in Premium is 38" versus 31" in coach and the seat width is 19" versus 17.1. It is not just a legroom upgrade that you can buy for $150. You get your own arm rests, a foot rest, a seat that reclines further, better food, free drinks, two checked bags, priority boarding and baggage handling, all for a price of course. A few years ago it seemed to cost about twice the price of coach, but this year the prices are more like $1600 and up. Not sure why.
FYI - If you know what plane you are flying, you can use seatguru.com to find out what other travelers experiences have been. It helped me avoid what looked to be an good seat, but turned out to have cramped legroom because of the way the bulkhead jutted into the cabin.