We visited the Bom Jesus do Congonhas from Ouro preto and we definitely found it worth the trip. We arranged a taxi (taking the bus is not really practical) from Ouro Preto, the trip takes about 1h20 minutes. You can visit the church in about 1-1.5h.
The church is well maintained and, wether you are a fan of the style or not, 'The Passion' is an excellent example of Brazilian baroque art. The fact that Aleijadinho, the sculpter, lost his hands and had to rely on his (slave) apprentices makes it all the more impressive.
The soap stone saints outside the church further exemplify the genius of Aleijadinho.
The church itself is again a nice example of the art at the time. No photographing allowed inside.
Make sure you don't miss the small building on the left of the church. It contains hundreds of ex-votos or gratitude letters when a certain wish or miracle came true. Some of them date back to the eighteen hundreds. Most of these ex-votos are from people who fell ill and recovered when all hope was lost; people that miraculously survived accidents; barren women that still got pregnant; etc... . It helps to have a Portuguese speaking friend with you to translate some of these ex-votos.