I really like Rondo, but I am in the camp of being ready to trade him. I would love to get a couple lottery level picks or a Demarcus Cousins type for him.
By the time the Celtics could be back on the contention map in 5 years, Rondo will be past his prime.
I also have strong reservations of paying Rondo the max. He has had the potential to get 15 a night on a consistent basis, but he never got to that point. If you are starting a team from scratch, I don't feel good about paying max money to a moody guard who cant give you a consistent 15 points per game during the season.
But... if you can't get a great return on a trade I would keep Rondo, and be prepared to let him walk if he can get another team to pay him the max as a free agent. He is still a heck of a dynamic player, and worth watching for the duration of his contract.