Yes, the RoissyBus looks like it'll be the way to go. We'll be there the end of this month, and if it's raining then perhaps the comfort of a taxi will be worth it, but otherwise it's far cheaper to take the Bus and a short enough walk to the PH. Thanks!

I'm now wondering if it's worth buying the Navigo Découverte pass which will cover the cost of the RoissyBus both ways during our 5-day trip. A weekly Zones 1-5 for EUR 34 basically gives us unlimited use of the Metro for an EUR 14, about the same as buying the 10-pack Carnet.