I've been using GIMP since 2000 or so.

For what I do with photo-manipulation, texture-building, or with postwork, GIMP gets the job done, and it does so admirably. It's also come a long, long way from the rude and crude beginnings that it had.

Now if you're a pro artist, Adobe still has a lock on that world. If you want to do art for a living, learn Creative Suite (or, well, they're pushing to Creative Cloud nowadays, so...)

If you're an amateur or prosumer just starting out, you'll have to decide for yourself, but on the amateur level, GIMP gets it done nicely.