I'm no comics illustrator, hey I've not even had the courage to enter an art competition (my avatar being my sole announcement of any artistic pretentions) but I quite like Gimpshop - easy to use and instinctive, highly useable features and nice, hand-done feel to lines (too much in my case!). It's also stable and (fingers crossed) I've not lost anything yet, nor has it caused any crashing. So thumbs up there too.
I'm a fan, but then I've had limited experience of other art programs. And being free, it gets an extra star.