It all depends on what you need, and what you're used to. Unless you're doing pre-press work and need to produce multi-layered Photoshop files for your job, you probably don't *need* Photoshop. I've never bothered to cross-upgrade my old Photoshop 5.5 for Windows at home since I moved to Macs three years ago; Gimp does what I need just fine (a little photo touch-up, rearranging screen shots, converting and tweaking graphics for the web).
If you have to work with Photoshop files that you get from someone else or ship off to someone else, it's nice to be compatible. Gimp will read many but not all .psd files, but as I recall can't write them.
Photoshop's text tool is still much much better, and the adjustment layers are pretty cute, but again it all depends on what you're doing. Gimp's very full-featured if you don't need the fancy CMYK color-matching stuff, so if you don't think you're missing something, you're probably not.
Of course, some people also can't stand Gimp's interface; if you're used to Photoshop and don't want to play the learning curve, shelling some bucks to Adobe might be worth it to you. If you're used to Gimp and like it, that's not going to be an issue for you.