The first CAT software I learned (as a transcriptionist) was a DOS-based version of AristoCAT ... then we upgraded to the Windows version, so I also learned that.
I later began going to court reporting school, and our labs had an early version of Case Catalyst on the computers in the lab, along with something called RapidWrite Pro (which has since been rolled into Case). Didn't get too familiar with RWP, as the version we had had a serious bug in it: If you hit more than 64 strokes and delete one, the software would delete the word you wanted to delete, along with half of the word before that one!
It took six months of complaining, but the school's labs finally moved away from RWP to a newer version of Case ... but by that time, I had already purchased my own writer and laptop, and my digitalCAT CD was on the way.
As for how I learned it, I learned the basics from the QuickStart guide ... then I looked around in the manual for the heavier stuff (commands and etc.) ... and I think (it's been a while!) I learned about dictionary manipulation on my own.
I don't think my exposure to the other CATs hurt me at all on learning DC ... but on the other hand, my exposure to DC has given me a good yardstick to use to check out the other CATs. Stop by ol' Cheap and Sleazy sometime and read what I wrote about TeLiTor when you get a chance (check the 01OCT06 update and the27JUN08 update), and you'll understand what I mean!
As for "DC Made Simple," that wasn't around when I started, but I did help with the formatting and layout! It's a good resource, and well worth checking out if you're having trouble with DC.