I actually just talked with somebody else about this today . If you are interested in making lots of art, these things can make your life way easier in the long-term. Be warned though, it takes a while to get used to. I say that if you think you'll put it to good use, it's worth it.
It depends what your are doing. If you can do everything you want to do with GIMP, then Photoshop is not worth it. For the most part I agree with marius_3_3_3 but I do disagree with him saying that it has a lot more features than GIMP. I would say about 92% of the things that you can do with Photoshop, you can do with GIMP. The only problem is that GIMP has a worse user interface and it is trickier to do some of the actions you can do in Photoshop.