If I know Microstation and AutoCad how hard will it be to learn Photoshop?

I have several years experience with Microstation and AutoCad in their mechanical drafting capabilities and am quite computer literate. I'm looking at learning photoshop. Will I find it at all similar to Cadd, or is it completely different? I'm just wondering if I could learn it by playing with it, or if I should plan on taking cles (I'd like to be really good at it - not just able to touch up photos). Thanks