I'll be the first of many to say, WAIT. Easy enough to pick up at the first museum (especially if it's a smaller one).

~Liz

Agree wholeheartedly with Liz.

Why would you want to do that? I can't think of any reason, it will cost you more, you must know if you've compared prices so then it's really just your decision as to whether that cost is worth it to you to have it in hand immediately.

WAIT.

It costs more to buy it here.
It's something else to remember to bring along and pack.
It's a breeze to buy it once there.

Help. Going to Paris and my daughters have never seen outstanding art. Last time I was in Paris I was going to buy a museum pass at the modern art museum (name escapes me) and the line went forever. Saw cool art in free churches instead of standing in line.

If you buy a museum pass at a smaller museum, I know that is the way to go. Do you know a museum without a crazy line? (Staying in the Marais District, if that helps.)

The Conciergerie rarely has much of a line in my experience. You can also pick up passes at the tourist info booth at CDG.

If you do not need the pass on your fist day there, go to any museum shortly before closing time when there will be no line. Just purchase the pass and leave; the clock won't start ticking until you use it for entry to a museum or monument.

If you are near St Germain, there is never a line at the Delacroix museum on rue de Furstenberg, There are hardly any people there either and it is a pleasant venue on a pretty square. I never bother with the pass, You can avoid lines at the Louvre by entering thru the carrousel du Louvre shopping center. Too many museums in one day give me a headache! And my favorites are free..the Petit Palais and the Carnavalet