I’ve been using Gimp as my primary image editing software. I never used photoshop (and I don’t want to). The only thing that make using Gimp quite difficult is the user interface – the main application window is separated into 3 (sometimes 2) tiny windows. I used to promote the software to other users, but this one important feature has been missing since the existence of Gimp, cause many of them prefer photoshop.
Single window mode is available for Gimp (2.7.3). However this version is still in development phase, and it’s rumoured that version 2.8 will be the stable release for this most requested feature.