I will pretty much always use Photoshop, but with a recent hard drive crash and the disappearance of my PS discs I've been using Gimp, and I'm pleasantly surprised at how good it really is. Especially for being free.
Speaking of the hard drive crash, no matter what program you use don't just back your photo's up on your main computer. If you don't have an external drive then burn them to discs. It's a pain, but I'm glad I did it. I was in the middle of editing a wedding I did and if I didn't triple back up I would be in a hot mess right now instead of just finishing. Off topic, but bringing it up made me think about it.