Using the RATP Bus is easy and convenient especially if you buy a 1 week or 1 month Navigo Decouverte, but event if you use a paper ticket you can still transfer from bus to bus within 1-hour and a half of the tickets first use.
We have found that most of the iPhone apps for the bus have their limitation so we actually download the Bus Routes from the RATP web site for the bus lines close to our apartment. We found the problem is finding stops when there are one-way streets, example: you got off the #63 bus on Boulevard Saint Germain des Prés headed toward Gare de Lyon, however you want to get the bus back on or about the same location, so you need to walk to rue Saint-Sulpice to the stop Eglise Saint-Sulpice to get on the bus #63 to Porte de la Muette.
Here is a link to the RAPR Site: http://www.ratp.fr/informer/pdf/plans-bus/plans_bus.php simply enter a bus number where is says: Afficher un plan de ligne de bus and select from the route from the drop down menu and click “Valider recherché”
Then add them to iBooks on your phone and they are available without internet access.