I've always used Photoshop as...well...it's what I've always used.
Someone waxed lyrical to me about the speed and ease of Xara Xtreme, tried the demo but think I'm too old in the tooth for learning new stuff.
Photoshop just seems intuitive to me, although I did learn a hell of a lot when I did a HND in Digital Photography.
If you are serious about using it, it is worth taking a course, if only for the fact that having to do your homework focuses you on learning new things, not just something you need for a particular job.