It depends entirely on what you want to see and how long you are willing to wait in line to get in. I was there on the 1st Sunday in August and, against all warnings, decided to take advantage of the free museums. I got in line for the d'Orsay about 45 min before opening and waited an add'l 30-45 min or so to actually get in after they opened. I spent about 2 hours there, then went and sat in an outdoor cafe for an hour having lunch and watching the passing parade. After lunch (about 2:00 pm) I got in line for the Orangerie and waited in line for about an hour. I spent about an hour in the museum. Contrary to what I'd been told, once I got in the crowds were not overpowering and I was able to comfortably see everything I wanted to see. I've had issues with worse crowds in the Louvre not on a free day. I had no problem waiting in the lines for either of these museums and I thought the day was well spent. I think the biggest drawback is that you have to limit yourself to two places and you have to be willing to wait. So, if that is worth it to you, I say go for it. If you don't like standing in long lines for over an hour or more then it's probably not the best use of your time.