My only issue with GIMP is the user interface, this isnt to say it is bad merely that I am not used to it.
http://grimthing.com/archives/2007/0..._vs_Photoshop/ is an article that does a reasonable job. It made me think, do I really need Photoshop? Give it a read, the author does use some language that may not be appropriate for young children but he raises some very good points.
This being said, I shoot with a DSLR in RAW format, GIMP supports 8bits per channel, my raw files are 10bits per channel. If you, or your team, shoot using DSLRs in RAW format this may be an issue if you want to work directly with the RAW files.