Pantheon was closed during our trip in Paris, so unfortunately we did not use our tickets. We had walked all the way over there to find lots of construction and no signs. I checked the Pantheon website and it said closed until July 1. I thought maybe Viator would make us aware of this prior to our trip, but perhaps not their task. Make sure you check the website prior to purchasing.
Skip the line pass wasn't necessary for the time we went. It was nice to show up with tickets in hand, especially since this was the very first sight-seeing adventure we did once we got to Paris. We went on a Sunday morning and there wasn't really a line to get in, but I imagine that at times, there might be. If you are on a tight schedule, this is a relatively inexpensive skip the line pass and could save you some time. I'm a planner, so having everything beforehand gives me peace of mind.