From the CDG, Rick suggests the Roissy bus to the Opera Garnier and then the metro to the rue Cler area. Others have suggested a route on the RER B thru the metro to rue Cler. Which is most the convenient and less hassle?
I'll add some details to Eric's and Swan's comments. There are two RER stations at CDG. CDG 2 TGV is directly below Terminal 2. CDG 1 is next to Terminal 3. To get there from Terminal 1 take the new CDGVAL driverless shuttle train which has replaced the inter-terminal bus.
IMO, the Roissy bus is the more convenient of the two to get to the Rue Cler area. The bus leaves immediately from the CDG terminal area (as opposed to the short bus trip from the terminal to the train station for the RER B).
The bus also drops you off at the opera area which is adjacent to a metro stop on the number 8 line which takes you to the Rue Cler area. If you take the RER B, you have to change metro lines at least once depending on how you go.
Either way is fine, and the bus could of course encounter traffic which the RER trains don't.
Which is more convenient depends upon where at CDG you arrive. I flew on EasyJet to 2B and hiked about a mile through the airport before finding the station for the RER. I passed several Roissy buses along the way. I wished later that I had hopped onto a bus to Opera and caught the Metro from there (line 8) to Ecole Militaire and the rue Cler. My previous experience was flights to terminal 1, so I was familiar with using the RER from there. Once I reached the RER station (2D?) it was easy to buy a ticket and board the train.
I had the same question until I did some internet research this morning. My hotel is near the Louvre, so taking the RER B can take me to the Chatalet/Halles subway stop instead of the Roissy Bus stop at the Garnier Opera house.
So my walk should be quite a bit shorter.
Do a search on "RER Paris" and "Roissy Bus Paris" and you'll find maps and explainations showing stops, etc. so you should be able to make an educated decision.
Good Luck
The key here is the time of day - and which day of the week you arrive into Paris.
Mondays are nothing less than hell getting into Paris in the mornings and you may find yourself sitting on the RossyBus for more than an hour if you are arriving prior to 9am nearly any weekday. I would highly recommend the RER during such times.
Either is easy and efficient (with the above situation) UNLESS there is a strike on such day.
If you have a lot of bags, people often find the bus easier with the short transfer.