Especially the older versions of PSP (7,8... while it still was made by Jasc) were great for pixel art editing.
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Pro Custom palettes available
You can work with a pre-defined number of color palettes, save and load custom palettes, and access several modes to reduce the number of colors on a picture.
Pro Several file formats supported
Paint Shop Pro can read and write images in several file formats: PCX, PNG, GIF, CDR, PSD, JPG, EPS, TIF, TGA, WMF... even RAW.
Pro Offers informative tutorials
There are quite a few informative tutorial videos available for users on Paint Shop's website.
Con Windows only
Paint Shop Pro is available only in Windows.
Con PSP8 has a relatively slow startup
PSP 7 worked better than PSP 8 in terms of startup speed. There are still versions around somewhere, if you look hard enough.
Con No more updates available
Paint Shop Pro 7 was pretty stable, but its creator JASC Studio is no more. It has since been acquired by Corel, and rather than improve Paint Shop Pro, they have killed it.
Con Windows Vista and upwards have limited support for colors
The main issue with PSP is that Windows is forced to "change" your window's color scheme while the software is still running. This causes screen flickers and odd color effects.