You know what I love? The fact that I can publish my university schedule to iCal, and see all my lectures in my calendar - on my phone, my laptop, my desktop and so on. But you know what I hate? When I decide not to attend a lecture, I can't remove the event from my calendar.
Now, the reasonable approach would be to just accept this (or to attend the lecture). Instead, I've built cal.alexbaron.me. After you sign up, you can insert an iCal URL, and start removing events based on filters. Then, just subscribe to URL generated by iCal Filter instead of the original RWTHOnline one, and you're done. Of course, this works for other sources as well.