I learned pretty much by getting it apart and back together. I have repaired successfully both of my DSLR's, two of my EOS 35mm's, an EOS flash (the 540ez I'm using now) and one of my 35mm Nikons, plus a few lenses from each.
However my Canon L glass I send that off to the professionals. Could I repair it, yeah I could. But F that, I paid thousands of dollars for some of those, I don't want to make them worse than they already are. If I paid 1150 for a lens, I can afford a couple of hundred dollars to properly repair it, from a trained professional who fixes hundreds of these things a week.