I'm not sure I entirely agree with Covoxer about Gimp. I personally find it much more user friendly than PS, I suspect the biggest problem is it doesn't do things in the same way as PS (and some people don't like the proliferation of windows). I also have never noticed any slowness, but this may just be because I've always run it with plenty of memory.

Having said that it's one huge deficiency is that it doesn't support 16bit images (yet). This is a real showstopper when you're working in B\W and mucking around with the exposure and curves. This issue can be worked around to a certain extent by correcting the exposure and applying any curves when importing from RAW.