I've used Paint Shop Pro since V.9 as well as Animation Shop 3 (it no longer comes with PSP, since Corel took over Jasc's product; you have to purchase it separately.) I love it; since V.10 they've included a nice learning center that you can use while working. I highly recommend PSP for anyone who wants a quality app. at a fraction of the cost of Photoshop.
I do intend to make the move to Photoshop one of these days (when I can justify 500-600 dollars.
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I've used PSP on all our work, from our website to our models packs and even in all our games. It's a great product, with plenty of support around the web.
I don't plan on upgrading to PSP XI though, as it doesn't add enough features for me to justify the expense at this time. I'll wait to see what they come up with next.
I suppliment it with a few filters from AlienSkin, which really add to it's usefulness.
I also know of a free program that will convert all those free PS brush packs, into .png format; so you can use them in PSP (especially all those nice grunge brushes floating around the web).
-Keith
Incidentally, my signature below was made in PSP with a few AlienSkin filters.