I use Photoshop at work and for my personal projects so obviously that is what I prefer. I did install GIMP though and I have played around with it and it seems very capable (infact, for the price - that is FREE - it's pretty phenomenal). Also, I learned that if you want it to have a layout more like Photoshop, there is a mod that does that called GIMPShop.
But I totally second what Midgardsormr said. If you don't have access to Photoshop and you're on a budget, buy a tablet before anything else.