Here's the thing about the GIMP UI... it's modular. It was written for us oddball penguin types, who just think differently than most folks (although honestly, we're trying to work on that). However, as with most applications written with Linux in mind, the vast majority of the user interface is customizable. In fact, while GIMP doesn't put out a 'photoshop-equivelant UI' for legal reasons, there's a third-party GIMP UI that is... pretty much identical, including hotkeys.
You can find various 'theme packs' for GIMP that work better for those familiar with and used to Photoshop.
Having said that, if you already HAVE Photoshop, and already familiar with it, there's zero reason to up and switch to GIMP. But for those who don't have the cash for a professional license, GIMP is a free to use, free to distribute alternative that can provide a very similar experience.