Kapu is one of Linz's alternative cultural addresses, better known for concerts, club nights, and independent programming than for polished institutional culture. That makes it a useful counterpoint to Ars Electronica, Lentos, and the official theatres: Linz has a sharper, noisier side too.
Whether it belongs in your evening depends entirely on the programme. Check listings first, then treat it as a way into the local music scene rather than a sightseeing stop. For travellers who like small venues, it can be more revealing than another grand facade.