The answer is - it depends. Depends on a bunch of factors. Not just the airline, not just the aircraft, but the age and version of the particular plane you end up on, what row and where in the row. I flew Delta on four routes in December and each plane was configured just a bit differently in PE/PS. The seats certainly don't lie flat, and don't go almost flat, they probably recline a bit more than a regular seat. What you're mainly paying for is better legroom, and a slightly wider seat, and fewer seats around you. And believe me, it's well worth it! You are also supposed to get reserved space in the overheads, although I doubt if that is strictly enforced. If you are looking for a bed, you need to move up to business.
On one of my legs I was sitting in one of the middle two seats and unfortunately the space under the seat ahead of me was filled with an IFE box. But the seat was so far away, it turned out my feet didn't need it. Had to stow the backpack overhead for the whole flight, though.