My own math background is through Calculus II right now, so I'm a beginner, but I can't say the Companion (though it is excellent) is something I could 'get through' in the sense that people usually mean it. By necessity it is terse, despite its massive size. I personally find it useful as a survey on the field, but I wouldn't recommend it as a teaching guide in and of itself for people who happen to be as dumb as me.