Well putThis question reads "spend money or not ?"
Photoshop remains vastly superior despite impressive features in Gimp and the ablity to use many (but not all) 3rd party filters desinged for Photoshop.
But the mere fact that Gimp makes using a freeware alternative to Photoshop a matter for serious consideration is testiment to how far the product can be taken.
There will always be things you can do with Photoshop that you can't do with Gimp. Do you need those things ? and are they worth the money for your individual goal ?
Spend the money if you believe you'll be using bleeding edge features and need to learn the tools that provide them. If all you want is great raster image software, try Gimp; chances are its limitations won't matter.