We sleep on a full. We could fit a queen, but decided not to spend the money on it. Why not? Because as soon as we get a place big enough for a king bed (next year), we’re having a king bed.
This is the wrong crowd to be asking, but in general, go out and ask people if they would ever sleep on a twin XL again, if they were never going to share the bed. Most would say, no, never. A king is the same width as two twins put together. I cannot wait until I have a king sized bed—when I get to sleep alone in our full, I sleep much better. When we sleep in a hotel in a King, I sleep much better. When we sleep in a hotel in a Full, I sleep about the same. The common thread here? More space=better sleep.