I’ve used the Skycouch on the AKL–LAX route, and on the return leg I flew in Premium Economy.
For such a long flight, being able to lie down made a world of difference. While you can’t stretch out completely flat, you can bend your legs and sleep on your side just fine. The space is surprisingly wide for an airplane seat. You can also sit upright against the wall with your legs extended, which is an ideal position if you're using a laptop or tablet for entertainment. I was traveling solo - as mentioned above, it would not be possible for 2 adults to lie down.
Separately, some friends of mine travelled with two toddlers and booked two rows of Skycouch. They said it was ideal for the kids. Both adults sat in the aisle seats while the kids stretched out across the middle and window seats.
In comparison, Prem economy has other benefits like better food and service, smaller cabin and dedicated check in, but you are still sitting upright for the entire flight. Skycouch gets my vote.