As others have said, it's an easy walk from the Roissybus Opera stop to the Park Hyatt. It probably takes longer to mess with a cab at that point, and the metro would be utterly pointless. I walked all the way to a hotel on the other side of Place Vendome from the bus stop with luggage in well under 10 minutes. Of course, if I had the cash to stay at the PH, I'd probably just take a cab.

In fact, I'm surprised they don't have some premium car service.