By rotating GMs, does it mean having everyone involved run a separate game, or a shared campaign in which all the players take a turn at DMing the game? I have done both.

My core gaming group is fortunate to have four people who've been bitten by the DM bug for many years (and one more recently). We don't strictly rotate, with each running games as their enthusiasm and level of availability allows. Sometimes that means multiple games trading weekends, sometimes that means one person runs for a while as the only one.

However, one time I did also have a game with two other friends where we traded DMing back and forth for the same game. Whoever's turn it was would prepare a sort of mini-adventure and let their character take a little bit of a back seat when it came to decision-making for the group that session. It was a fairly combat-heavy game with simple stories so it wasn't difficult to trade back and forth.