Milla G.
Yelp
This site was more interesting AND less crowded than most of the archaeological and historical points of interest in Rome. We got there less than an hour before closing, and wished we could have had longer to explore- as well as a tour guide or Audioguide, though I did try to follow around an English-speaking tour group discreetly in one of the underground levels.
The church is fascinating because it has 3 levels - and each of the underground levels was discovered from THOUSANDS of years apart in history, with insanely old structures and even an underground Roman home with running spring water (two levels underground) from the 1st century.
For 10 Euros, it is a more interesting ticket than many of your other options in Rome. If you have already hit Colliseo, Roman Forum, Sistene Chapel, Vatican, Trevi Fountain, and St. Peter's, this might be worth it for another stop.
NOTE: We did not get to do the Underground Rome tour or the Underground Colliseo tour because it was all booked - so it is possible that if you have done either of those, or the underground Crypts, that this might be similar to those - but I am unable to provide a reference point as we did not do those. It was a lot more interesting than underground at St. Peter's where the Pope's were buried, because it is archaeological ruins rather than preserved and modern.