You should definitely visit the separate chapel, very beautiful. In the cathedral you can see the chair of Calvin which is very small. I enjoyed the view from the towers of the cathedral. It is a climb, but worth the effort.

Information
Saint-Pierre Cathedral
Place du Bourg-de-Four 24
CH-1204 Genève
Switzerland
http://www.saintpierre-geneve.ch/
https://en.wikipedia.org/wiki/St._Pierre_Cathedral

Visited November 2012. You can find my review about Cathedrale de St-Pierre also on TripAdvisor.