We tried ds in a booster as he was turning 5. He sits really nicely and is developmentally ready for a booster. But, he fell asleep in the booster after 5 minutes in the car, and flopped right forward. My other kids flop against the sides, and it was never an issue, but this kid was really not safe when he was asleep. He also sleeps soundly in the car, so even though we woke him up, he went right back to sleep and flopped forwards and to the side.
DH was happy for me to spend the extra money on the Nautilus when he saw that, and we installed it this morning. Although ds was very happy with it when he tried it in the store a few weeks ago, he didn't want to go back to a harness after being in the booster for a couple of weeks, so we told him it was like what astronauts sit in for blast-off. Height-wise he'll fit harnessed for a long time (my8yo ds still fit), but the crotch strap may only last him for another year or so.