Overall, this is a a pretty worthwhile site to see if you are already in Lima. It doesn't rise to the type of attraction that you would go out of your way for, but if you are in Lima, this is worth seeing.
The architecture from the outside is actually fairly stunning. The church and catacombs are situated near the Plaza de Armas in Lima, so making a stop here to check out the church even just from the outside is worthwhile because it gives you a chance to see everything around it.
There is a tour (both a Spanish and an English version). Admission and the tour cost 7 soles per person (assume about a 2.8x conversion rate, so this is just a couple of dollars each). The tour is actually pretty decent. Our guide was personable and funny if not a bit quick.
The woodwork on the ceilings is amazing and worth the price of admission on its own. The library features books that are several hundred years old. That room is also a stunner. The rest of the tour features paintings, artwork, woodwork, and a tour of the church.
The real star of this visit is the catacombs. Under the church, they served as a burial ground for Franciscans and Catholics. The tour is interesting. The bones are creepy. If you are very claustrophobic, you might have issues in the catacombs. But if you are only moderately claustrophobic, you should be ok.
Note that you cannot take photos inside the church or the catacombs, so this is one of those things you look at but cannot capture. Also note that they churn a bunch of tours through, so each one is really a sprint through the facility. There is not a lot of opportunity to stray and really look at things that interest you. The total tour took about 50 minutes for us. It seems like the tour length is directly related to how many people they have backed up (more people means shorter tours).
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.