The 4 or 6-day passes are quite worthwhile if you visit an average of one museum per day and I highly recommend them. The 2-day pass is worth purchasing only if you visit 2 museums per day, which might be too much for some of you. I mean two museums in one day, ok, but four museums in two days? Your call.

In all cases, there is no need for you to buy them in advance (and pay an extra 12€ for delivery to your hotel, or having to go to the main Paris Tourism office to collect them as you will lose some of your precious time doing that). You can easily purchase them in a lot of places in Paris with no line. I can even buy them for you (to save you the hassle) and give them to you in person when I see you. For this service, I will invoice you the official cost only as favor to you, not like those skip-the-line sites galore whose only income comes from the (sometimes exorbitant) commission they make from those transactions.

Now, about skip-the-lines tickets (and the lucrative business around them), here is what you need to know:

In almost all cases (except Eiffel Tower, Louvre, Versailles, the Catacombs and the big temporary special exhibitions), they are just prepaid dated tickets that you can easily buy yourself online at no extra cost on each museum's official website. Those prepaid tickets will allow you to use a special entrance which is the same one used by people with museum passes but, in all cases, you will still have to line up for the security check which is unavoidable!

Nevertheless, I highly recommend buying museum passes or paid-in-advance dated tickets because:

1) they permit you to use a special entrance (where the line is much shorter than the line for people who don't have tickets)

2) once inside, you don't need to line up again to buy your ticket!

The Louvre and Versailles are unique (of course!). The Louvre now forces you to choose the date and time of your visit, while Versailles gives you more options (which aren't necessarily better. This could be a good time for you to pour yourself another drink...).

For the Museum passes, you have 3 options:

So, here we go: due to the fact that both museums suffer from overtourism, since last year (2019), in order to limit the number of regular visitors during the high season, the Louvre now prevents you from buying last minute tickets on the spot and forces all visitors to buy their tickets in advance. And, even if you have a museum pass, you still need to book (at no extra cost) the date and time of your visit in advance here otherwise they can refuse you access to their collections. And to book your time slot online, you need the serial number of your museum pass... that you can have only if you have your museum pass already in hand because "you can only book your visit at the reception of your Paris Museum Pass. We are not able to give you the serial number of your Pass in advance" as it is stipulated on Paris Tourism's Office official website! How silly...

So, to make things even more complicated for tourists (right, Katy?), if you're interested in museum passes and you want to go to the Louvre with them, you either need:

1) to have them sent to you by international registered mail (which is expensive and antiquated),

2) to have someone like me buy them for you and make the bookings for you,

3) or you need to pick them up right upon arrival and make your bookings for the Louvre right away! There are almost always time slots available for the next day (even though it could be in the afternoon only) and you should have almost all choices available if you book two days in advance; still the efficiency rate of this procedure is rather poor to say the least...

And for Versailles, get ready for this, people with museum passes cannot book time slots: they have to join the long and slow-moving line for people with ordinary prepaid tickets to get inside the palace. And if you want to join my guided tour with your museum passes, they will force us to buy at least two on-the-spot entrance tickets in order to use the group entrance. That means that, if you're a couple, your two museum passes will be of no use to enter inside as you will still have to pay for your entrance tickets again! How well thought-out that is... Your passes could still be useful for the Trianons and hamlet though, but not for the gardens when the fountains are on because you then need an extra ticket!  

One silver lining: if you have children under 18, they can go inside almost all museums for free without a booking. You don't need to book a free ticket in advance online for them for the Louvre. But they will need a ticket for Versailles if they are part of a guided tour!

In short, I do recommend museum passes EXCEPT FOR VERSAILLES where they are useless. Especially that since last year individuals can now book a time slot for Versailles if they pay €20 or €27 (depending on the day of the week) for their "passport with timed entry". It not only gives you access to the palace with almost no line, but you can also visit the whole estate and gardens (compared to the fee of €18 for whatever day of the week for the palace entrance only). So far, not too many people know about the time slot tickets for individuals at Versailles so there is almost no line there but, after some guide books (or websites like mine) will have informed visitors who go there on their own that, if you pay only two Euros more, you will save doing a two to three hour line and have more time to visit more of the estate, all individual visitors will buy time-slot tickets and they will then create a huge new line at the entrance for individuals with timed entries and then they will all be back to square one!

Let it be said at this point that the best way to do the shortest line and to enjoy a smooth visit at Versailles or the Louvre is to join a guided tour with a professional who knows the ropes and can use the group entrance with its own time slots! And I can only wish good luck to those of you who don't...

About the Eiffel Tower, which also gives time slots here, as they are almost all sold out months in advance just a few minutes after having been put online, some agencies make huge profits by selling those time-slot tickets at three or four times the original price. They justify their price by the fact that they provide you with a "guide" (most of the time non-official) who will give you (and the others) a little walking tour around the Eiffel Tower and tell you some anecdotes and facts about the Eiffel Tower while you do the line for the security and then he/she will leave you with a sorry face at the beginning of the line for people with time-slot tickets!!!

Now, some websites do have the honesty to tell you (at the very bottom of their webpage) that "even with direct access, you may have to wait in lines for security and for the elevators. In high season, total wait to access the 2nd floor can be up to 25 minutes. Summit ticket holders will have to wait in line on the 2nd floor to access the summit's elevators. In high season, this wait can be up to an additional 20 minutes." I agree for the 25 minutes wait to get to the elevator (not including the line for the security) but, in high season, the additional wait on the second floor to get to the summit can easily go up to one hour going up and almost another hour getting back down! (because the same people lining up going up will line up with you again going down...).

My advice to you: don't go to the very top of the Eiffel Tower because 1) you waste too much time 2) you are too far up to enjoy the view or to take good pictures. The best view is without any doubt from the 2nd level:

Anyway, whatever option you choose, whether you like it or not, you will have to do some lining up as it's the most visited monument in Paris and the elevators can only take so many visitors at a time. I say: just go early in the morning, be prepared to line up for about 30 minustes for the security, another 30 minutes to one hour to buy your ticket and reach the elevator, and then you just go with the flow! Don't forget to bring some stuff to keep you and your kids (if you have some) busy while you wait. And if you go in the afternoon (because of the time difference), you can call all your friends and tell them "hey, guess where I'm calling you from?"; or you can make a competition among your party of who will take the oddest picture of the tower. Honestly, it's not as bad as it seems... except during the very high season (around Easter and from June to August). And the good thing about getting there with no previous booking is that you can choose to go when the weather is at its best and you are therefore not forced to go on a day where it's raining cats and dogs because that's the date and time you booked months in advance and you can't change it!

And if you can afford it (ooh la la!): go for lunch or dinner at the Jules Vernes on the second level. They have their own private elevator which will take you up there in a couple minutes!

About the Catacombs, their official website does sell time-slot-skip-the-line tickets at more than double the original price which are totally worthwhile to avoid the two-to-three-hours-minimum line to buy tickets on the spot. You will find all the detailed information on this page (if you find the entrance...).