This is another one of the first Wildcards available in the game, and as the other ones is only usable once you discover Political Philosophy and adopt a Government with a Wildcard slot (unless you're playing as Greece or Poland). Compared to the other available cards - Revelation and Strategos, Inspiration is maybe the most generally useful, since it helps you get a Great Scientist early in the game. However, think carefully if you want to ensure your getting a Religion - the AI is pretty competitive on religions, and if you decide to go with Inspiration you will get realistically 2 - 3 Great Prophet Points (assuming you've already built a Holy Site with a Shrine and have adopted Classical Republic) to other civilizations' 4 or more, and you may end up without your own Religion!