In my opinion GIMP is like an alternative program, it is in no way comparable to Adobe Photoshop. Their goal purposes are different, which pretty much sums up their differences. Adobe Photoshop is for those that want to start or have a company, or somehow been able to get a hacked version of the program (keygen/cracks). Basically being a massively used program, used by individual designers as well as professional companies. GIMP on the other hand is cheaper (free), which makes it a much better choice for the starting-designers out there. Not just starters, also those that simply can't afford the program or somehow kept being attached to the program after using it multiple times over and over.
This pretty much means that I like Photoshop a lot more.
Just my opinion, don't think of this as an offensive act in any way.