I also have DD3 and CD3 (though I'm not sure how much I use the tools provided by those add-ons...its hard to tell...I mostly just draw shapes/add effects/drop symbols.) The youtube tutorials are essential and I haven't gotten the tome yet, but I plan on it. The most important thing is just making a bunch of maps and pushing to see what the program can do.
I've experimented with Gimp and Photoshop (prefer PS), but I find them to actually be a little less intuitive for me (probably just a result of me learning CC3 first). I wish I had the artistic ability to make my own symbols.