Hi all!
We just got back from an incredible trip to London, Paris, and Amsterdam. Before we left, we purchased the Paris Museum Pass online to pick up at the Tourism office in Paris. I would highly recommend picking up your passes the day before you want to start, as it took close to 45 minutes at the Tourism office to get the pass. I'm glad we did that, as we did not want to waste any time on days we were using the pass.
In terms of the "skipping the line" rhetoric that I have seen, this is really not the case at major museums! We got to the Louvre and flashed our museum pass to the worker, and were pointed to an equally-long, different line than those who still needed tickets. This was a line for security, which took approximately 45 minutes. Once we were through security, we were good, but the long wait was not something we were expecting. Also, we attempted to go to the Orsay at about 2pm, and faced the same issue, even with the "Museum Pass" line being MUCH longer than the other line. For the Towers at Notre Dame, there is not a separate line for Museum Pass holders, and when we got there first thing in the morning, we opted not to wait in the line that wrapped nearly around the block in the cold.
Now, I say this not to diminish the value of the Pass. We enjoyed using it and saw many of the smaller museums that we normally would not have in Paris. However, we did notice that the one way to bypass lines at both the Louvre and Orsay was to buy admission ahead of time online for a specific time. Those specified times allowed you to skip the massive lines and literally walk straight through at both those places. It's just something you should weigh in thinking about how you would use your Museum Pass. I'm definitely glad we got the pass, but had I realized about the lines, I definitely would have planned a little bit more.