We went to St Matthias church at the same time as we were visiting Fisherman's Bastion. We had brought tickets for both attractions. Fisherman's Bastion was ok but Matthias church is a definite must do. We looked around inside the church and then paid to go up to the viewing platform at the top of the church.
Admittedly there are alot of steps and they are very steep and not very wide, but wow, what a view you got from the top. You could see a complete 360 degrees of Budapest. A tour guide takes you up and stops at about halfway to explain a bit of the history of the church, he spoke really good English and confidently answered any questions.
If you are reasonably fit and don't mind heights, this is a definite must if you visit Budapest.