The four day pass is 50 E (maybe $65 or so Canadian) and is worth it if you will visit at least six covered attractions (there are 60 or so) during the four days. Louvre is 10 E, the Pompidou is 12 E, Pantheon is 8E, so that gives you an idea of if this is a good value.
It is good you did not buy in advance. They are available at most of the attractions that take them as well as other places. You have avoided delivery charges and markup.
Other advantages . . .
1. You avoid lines in some places. We found it especially useful for avoiding lines at the Musee d"Orsay and L'Orangerie.
2. It gives you the freedom to drop in at museums you might not otherwise have looked at. If you don't find the museum interesting or worth a lot of your time, you don't feel like you have made an "investment".
3. Even if you can't skip lines, there is still time and hassle saved by not getting out your wallet and buying another ticket at each attraction.
4. It is good for multiple entry. If you want to go back to a museum multiple times, you are covered.
The Paris museum pass would NOT work if you are the type of person who would go to a museum and spend the day. It also might not work if you are planning to spend a lot of time at sites that are not covered (but most of the majors are).