If you can find some sort of padding, like maybe a camping sleep pad, it's quite comfortable back there. You've got a bit of room to roll around in and the recline is much less than the front passenger seat. Plus, your legs don't dangle. I also find the seats nearly as hard as lying in the trunk, so a sleep pad would make the trunk far more comfortable than the seat.
FWIW, I found the OEM cargo mat to be quite squishy, so you don't necessarily have to remove it when you sleep back there as well. Unless it's an infant or maybe a toddler at most, it's strictly a one person deal though.
The oddest thing was I couldn't get the trunk to close with my remote. I had to hold the button on the door, which was quite an awkward stretch for me. Did I do something wrong? I've only got about 200 miles of seat time in the car so it's all still really new to me.