Yes...I have. You are right...it will be very hot but more importantly, terribly crowded. I would avoid visiting Rome in the high summer if possible, unless you relish the thought of looking at all those marvelous sights over the heads of about 1 million groups of schoolkids. If you do decide to go, get timed tickets for the honeypots sights, such as the Colosseum, Vatican, etc.
Take the train ride out to Tivoli Gardens ::::
http://en.wikipedia.org/wiki/Hadrian%27s_Villa