I had planned to take in one of the weekly organ concerts here, and what with one thing or another, we didn't make it to Haarlem from Amsterdam on the right night (I think it's Tuesdays at 8:15pm).
I was, however, pleasantly surprised to have the organ going full-tilt when I arrived the next day during the early afternoon. The organist was practicing bits and pieces of various Bach compositions, and the organ is indeed spectacular, both in sight and sound. Mozart and Handel played here, so, well, there you go.
The rest of the cathedral is beautiful with lots of great things to see... the carvings on the choir in particular are really beautiful.
Rick Steves has a nice little walk-through of the other things to see in the church in his Amsterdam, Brussels and Bruges book.
Even if you don't luck out and hear the organ being played, this is a must-visit attraction.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.