Museum pass - costs more, but save time?

Are you talking about the Paris Museum Pass - or some other pass? If it's the Paris Museum Pass, were you thinking of buying it for two, four, or six days?

Oops! I meant Paris Museum Pass. Will edit original post.
We're probably looking at two 2-day Museum Passes, just the way our trip is laid out. The attractions covered by the pass are at opposite ends of a week, so just outside of a 6 day pass.

Christine, which museums/sites are you planning on, because the time saving is good in some, and unnessary at others..Also for Towers of Notre Dame it does not allow line skipping. So it would help if you actually tell us where you want to visit in a two day period.
Do be sure you lay it out so that the sites you want to see are open, many if not most museums close on a monday or a tuesday.

What time of year will also matter. Traveling in off-season helps, but doesn't guarantee low crowd levels. I was shocked at line levels back in October last year. Apparently everyone else on the French fall school break had the same idea to hang out in Paris with their kids as I did.

Here's our rough itinerary. I planned it with time in mind, not thinking about how to use the Museum Pass. But if we can cut down on the lines, we'll fork over the extra money. I was thinking two 2 day passes for day 2-3 and day 7-8? Day 2 Su - climb Notre Dame Tower, St Chapelle/Conciergerie, organ concert at St Sulpice, Historic Paris Walk Day 3 - Sacre Couer, Montmartre walk, Opera Garnier, Champs Elysees walk, Arc de Triomphe Day 4 Tu - Napoleon's Tomb, Rue Cler walk, Rodin museum, Marmottan museum, Eiffel Tower at dusk/evening Day 5 W - Mont St Michel Day 6 Th - MSM, Musee d'Orsay at night Day 7 Fr - Chartres/Versailles
Day 8 Sa - Louvre, Tuileries, L'Orangerie

Christine , no offense please , but attemping Chartres and Versailles on one day ? I don't think that amphetamines that strong are available without an Rx . Please accept that my comment belies a somewhat twisted sense of humor .

Heh - it's okay, I know. I think we may just stick to Versailles that day and save Chartres for another trip.

Buying passes for days 7 & 8 makes sense. Passes won't allow you to skip the line for the tower at Notre-Dame or the security line at Sainte-Chapelle. There isn't likely to be a line at the Arc de Triomphe.

Christine, personally I wouldn't get a pass at all. I would buy my ticket for cost on the Versailles website and use alternate entrance at Louvre, and that with your itinerary are the only two places the pass would help you at all much anyways. I have been to Louvre over a dozen times, always in busy season, and have never had to wait more then a few minutes to go through security( unavoidable with pass or not!) when I enter via the underground Caroseul Shopping mall( off Rue Du Rivoli).
Line for Notre Dame Tower is completely unskippable even with pass you have to get in line, so go EARLY, before it opens to avoid long wait.

I'll take back what I said about buying passes for days 7 & 8. Follow Pat's advice.

Thanks for the advice everyone! I think we'll go with Pat's advice and just purchase individual tickets. We'll be at the Louvre on Nuit Blanche, so it's free then anyways (but may be mobbed).