In my experience, mostly essential. I've missed one or two lectures, and the only way I've found to catch up properly with the material is to copy out a friend's notes by hand (so that it does go through my brain) and normally ask them a bit when I'm trying out questions on it. Most of my lecturers didn't put their notes online, although for the ones that did I found this substituted for the friend's notes, but not for the conversations explaining the little things/steps that a lecturer commented on, but didn't write down. Unlike in arts subjects, your main source of learning is lectures, and watching a lecturer do the stuff, and hearing them explain it, is the best way to see material.