We loved having the museum pass the last time we were in Paris, just a year and a half ago. In five days we were there we used it many times, and it more than paid for itself. We were twice in the Louvre, twice in the d'Orsay, at the Orangerie, and at several other smallerl museums and special sights. I liked the fact that it made it easier to "pace yourself," for you could visit for just a couple of hours at a time instead of feeling like you had to spend the whole day because you had paid the admission price.
If your scheduled days in Paris have some free time for visiting museums, I think it is great to have. The next time we go to Paris, we will definitely get it again. We bought ours there in Paris, although I had researched it quite thoroughly before our trip so I would know about where to buy it and which one to get. I still have it, and sometimes I look at all the museums that are included on it and think about which ones I want to visit next time!