There are three programs I work with: Photoshop CS5, Gimp and Paint.net.
I each use them for a different task.
Paint.net for spriting and basic/quick editing of images (scaling, rotating, scewing and what not.). Opens up very quickly compared to the other two.
Gimp for Gif image editing and creating. Slow on starting up and I don't get half of the program. Everything seems to be done through a detour or something...
Photoshop CS5 for everything else. Slow on starting up, but it makes up for it for what it can do and what I need it for.
These three all allow use of layers, channels (Except for Paint.net) and transparency. Which I all use a lot.
So it all depends on what you want from it. I suggest you check out Paint.net and Gimp. I personally don't find Photoshop that suited for spriting though.