I'm a big fan of Photoshop, but I used Paint Shop Pro back when I was mostly on my PC and I know it was fine. Now that I've switched to Mac, PSP is kinda out of the question.
I got a real good deal on an older version of Photoshop off of eBay. (It was legal and registerable and all that.) I registered it with Adobe, and then could upgrade it later. It was a fraction of the full retail version's price.
I am sure PSP is fine, but there is SO much support for Photoshop out there (in the way of books, videos, even a conference, for crying out loud!) I like that built-in feeling of community and support. Plus, I find myself learning new things about Photoshop's depth all the time. I am glad I have it. You never know when you might need one of its more "hidden" features!