Photoshop is the only one that I use, I use it more for processing my Pics before they are put on line,
I bought my copy from a slighty dodgy source, but still has all of the features and with plenty of practice you can do some really wicked work.
There is a new version out for Vista, which will fully work with any add on which you may add at a later date (such as new filters, workspace additions etc)
One of the best features I use a lot is the grouping of images and editing them all at the same time, makes processing large batches of images a dodle