Is it necessary to book ahead? Read too many stories of people waiting forever for a paid shuttle that doesn't show or shows late. Can't I just book it when I arrive? Can't risk missing flight to Milan. Staying at pricey Sheraton on last night to be AT the airport for earl Sunday AM flight to Milan. Won't have cell or computer with me. Can I find a shuttle at CDG upon arrival? Will it cost soo much more not to book in advance?

Take a cab - easy. You will not find a shuttle on arrival unless you have booked in advance.
And double check at the Sheraton about their shuttle to be sure it starts running early enough for you to make your flight and that there is sufficient capacity that you will not have to wait.

I agree that taking a taxi is the best way not to wait -- the taxis are waiting for you.

Everything depends on how much you are willing to pay for convenience. (Since I am still an RER B customer, that explains all you need to know about me.)

ditto on the taxi

2 ditto's for the cab

Thanks for the advice. I chose the Sheraton for the last night as I was fearful of getting a Sunday AM 4:45 transport. Supposedly, the Sheraton is the only hotel actually at the airport at Terminal 2.

Actually, there are others but I'm sure you made the right choice

Dukey which other hotels are actually at CDG? I thought other (airport) hotels were 2-15 miles away. Ibis....

While not directly in a terminal, the Ibis, Novotel, and Hilton are all close to the automated CDGVAL that connects to any terminal.

YOu cannot book a shuttle upon arrival. They don't allow that at CDG, and I imagine it's due to some agreement with the taxi companies (otherwise, they would be in direct competition). They don't allow that at the airport where I live in the US, either, for similar reasons. They do in some cities, though.

Ditto on the RER (for me, anyway). If you think about it, it's just like taking a train to another city - you ride to town, get off, and find a taxi, bus, or Métro (which is included in your RER ticket) to your hotel.

Depends on your priorities.