Most tourists visit this church to ponder Bernini's "St. Teresa in Ecstasy," a stunning sculpture. If you go, take the time to look around. I was impressed by the statue that sits directly across from "St. Teresa" and had fun comparing the two. (The other is not Bernini's work.) There's a small gift shop. Check open hours before you go. A huge benefit is that large tourist groups don't come here, so it's usually not crowded.
Santa Maria della Vittoria is located just a short walk (less than 10 minutes) from the Termini, and easy to find with a good map.