I can't use the GIMP for a few reasons, one of which is my work flow is so hinged on Photoshop's way of doing things that I can't migrate. In terms of usability there are many areas where GIMP just can't compete in. If you're working on your own projects, then fine, but in a professional environment it really does matter.
That said, as far as I can tell aside from a poor but usable implementation of 32bit floating point images the only thing Adobe has added in each successive version since CS is more bloat and harmful UI tweaks. It's a bit like Office, really.
The reason people pay for it is because they know they can turn right around and charge their customers for having it. The people that use it do so because of style over substance. They know that their clients want someone that uses Photoshop not GIMP or some other program they have never heard of.
This is absolutely not true. Our customers have no idea what software we use to make our products. Why should we tell them? What would they care? We use Photoshop because... and you have no idea
how much it pains me to say this... it's the best tool for the job. They simply have no viable competition that can do the things that they do. I can't stand Photoshop (and generally have a great deal of ire for anything Adobe spits out) but it's simply the best that there is. Unfortunately.