I've been using Gimp for ages (15 to 20 years) and it was always solid for me. Maybe I was just using the more stable features? But I've mostly used it on Linux. My major gripe was the Save / Export fiasco [0] but even that is OK now thanks to Saver plugin [1].

[0] http://www.gimpusers.com/forums/gimp-user/14339-hate-the-new...

[1] http://shallowsky.com/blog/gimp/saver.html