I guess I would say that it all depends on your feelings about lines and crowds and your level of patience when dealing with them. I was in Paris on the first Sunday in August, which is one of the least crowded high season months in Paris but still crowded with tourists, and I managed to see two of the more expensive museums on that day without too much hassle. The lines for both were long but I was in line for the d'Orsay when it opened so was in before the crowds were unbearable and enjoyed some relatively uncrowded sections and was able to see all I wanted to in about 3 hours. It wasn't so bad that the crowds were shoulder to shoulder, or anything like that, but there were a lot of people there, although I will say that I avoided the museum cafe because it was jam packed. After a 2 hour break for lunch and just strolling around I got in line for the Orangerie which was about 45 minutes of waiting but worth it, I was able to see the Monets without being jammed in too bad. At least the day I was there the other people were very polite and there was no shoving or pushing in line or to get closer to the art works, etc. So, all in all, my experience wasn't that bad. I did heed the advice I had read here and avoided the Louvre on that day so I can't speak to that but I'm sure it was pretty bad from all I've heard.
So many people have no patience with waiting in line or dealing with crowds but I guess it just doesn't bother me as much as most. I look at lines and crowds as just something I have to put up with in order to see the 'must see' places on my personal bucket list, especially because I need to skimp and do my traveling on a shoestring budget - it's just the way it is.