While planning for our trip to Paris, I read a few negative comments on the Paris Museum Pass (PMP) and not many details on how it works and reservation requirements so I thought I'd post this here for others who are searching for answers.
We decided to purchase a 6 day pass for 110euro each. We were very satisfied with it and for our purposes it was well worth it, but your mileage may vary - We stayed 6 nights near St Suplice in the 6th, Paris was our only destination for this trip, we decided to concentrate on museums and monuments since we were there during a 'relatively lower tourist time' (still busy but not crazy).
From the PMP website, you can find the list of museums that require reservations. We checked several times in the 2 weeks before our trip and there were lots of time slots open for all places. It's not clearly explained but each site that requires reservations makes you set up an account, but making a reservation doesn't require you to already have a pass. If you wanted to, you could make more than one reservation with different accounts, there's no money involved in making the reservation (unless you order an audioguide at the same time). For sites that are part of the Centre des Montuments Nationaux, use the same account, but log in first, then go to the site to make the reservation (it's easier). You don't even have to buy the pass until you arrive.
The day before we left, we booked timed reservations at Ste Chappelle (Thur morning), Versailles (Fri), Louvre, (Sat morning). Each reservation came with a pdf and/or mobile pass that can be added to google wallet. We didn't print out any pdfs.
The morning that we were flying to Paris, we bought the PMP online, and printed out the pdf ticket (we also stored it digitally on our phones)
It is true that the PMP doesn't let you skip security lines but the longest we waited was 30 minutes at the Louvre (got in line at the Pyramid entrance at 9:30 for 10am entrance, was through securiy and past the museum entrance by 10:02). For most sites we waited less than 10 minutes to get through security and having the pass made it faster, especially if there was a timed reservation. We never had any problem showing and scanning our PMP. Some places scanned the reservation as well, others didn't.
Here's where we went using the pass:
Wed: Musee de Cluny, Pantheon
Thur Ste Chappelle (10am res), Conciergerie (we got in without reservation even though it was supposed to be required), Musee Picasso
Fri: Versailles, palace and Trianon (10am res for Palace, noon for trianon, but the timing for that doesn't matter, it opens at noon)
Sat: Louvre (10am res), cite Des Science Et De L'industrie (least favorite), Musee Rodin
Sun Musee D'Orsay, Arc Di Triomphe (spent much of afternoon at Pere La Chaise cemetary then the sun came out so we went to Arc Di Triomphe before sunset, with ticket security line was about 15 minutes)
Mon Musee Delecroix (very small, need 30 min max) Centre Pompidou, L'Orangerie (16:45 reservation, made Sat evening, we should have booked this earlier, there weren't many time slots available).
So, as you can see we went to many museums during the 6 days of the pass, and for us it was worth it.
Happy to answer any questions.