The thing with Photoshop is that I don't think anyone can truly master it.
You can get people who are really, really good at it. But the program is deep and complex, its impossible to know everything about it.
I've been using it for about 2 1/2 years and I've only scratched the surface of it. I don't use it Hardcore, I mostly use it to manipulate images for websites.
The good thing about Photoshop is, it doesnt matter what you want to do with it... theres probably somebody posted a tutorial online for it.
Best way to learn, I think, is to get familiar with the basic concepts and learn where everything is. That way you can at least understand what people are talking about in tutorials. Some (most) of the tutorials online assume that you have a basic knowledge of the program, like how the layers work, gradients, masking ect.
Yeah unlike some people I never learned anything about Photoshop in school and never had anyone show me anything. People here have been very helpful in getting me started though and pointing me toward tutorials and giving me tips. I appreciate it. I don't try to do anything complex really, I just have fun making sigs.