Not very prepossessing from the outside as the main tower is clad in scaffolding and plastic sheeting at the moment but once you go inside it's a lovely church.
It's free to go in but The Adoration of the Mystic Lamb altar piece is a small fee which I think was covered by my CityCard. It's well worth the money though, as another reviewer mentioned, some cleaning of the glass protection would be good. There's a small bench and plenty of standing room but, luckily, there were very few people when I was there. A couple of the panels are being restored at the moment and you can see them actually being worked on at the Museum voor Schone Kunsten (Fine Art Museum). It's an ancient and stunning piece of art.
The downside - the frequent notices saying 'no photography' versus the large number of visitors taking pictures with flash going off continually whilst the cathedral staff just watched. I didn't take any photos as instructed but I would suggest that they either remove the signs or police the instructions as I did feel somewhat resentful about it as I glared at the photographers.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.