I've taken advantage of most of the things listed in other answers but there were a few which I didn't see highlighted...
Really flexible keybindings
Also called keyboard-shortcuts or 'accelerators' depending on where you're from, I found that – at least in CS3 – Adobe arbitrarily limited which key-bindings were allowed for certain functions.
The specific example I remember is that CS2/CS3 considered Full-Screen Mode to be a "Tool" which could only be bound to a letter-key like F, rather than something more standard like the F11 key, which was not allowed. This was one of the things that tipped me toward GIMP.
Subsequently, I've taken advantage of 6 hooks for "increase/decrease current foreground swatch hue/value/saturation" which are really useful for some tasks, especially painting.
No-fuss portable settings
A big host of settings in GIMP including tool defaults, brushes, and even scripts and extensions can simply be placed in one's
~/.gimp<version>/ directory (or
%USERPROFILE%\.gimp<version>\ in Windows), and ported between different workstations using a USB-drive or using Git. This is nice if you use GIMP both at home and at the office, since once the file-links are set, settings are picked up automatically so there's no remembering to click on Import/Export Settings when sitting down or getting ready to leave.
Community and source-structure
This is a general advantage of open-source softwares; if you or your company want to develop or pay to develop/improve GIMP's functionality, this can be done and the resulting code is then (per GIMP's license) given back to the community, which might maintain it for everyone in perpetuum for free (if it's useful).
Looking forward, updates to GIMP tend to be focused on what users need, rather than what might allow a proprietary-software company to charge more, lock its users into their own software suites, or otherwise make money.
In many cases, it's also possible to find out exactly how and why something does (or doesn't) work straight from the developers themselves. This can help to inform decisions about which fixes or improvements might be worth the cost of contributing, if you're using GIMP as a part of your hobbies or business.
Many of the answers (and a few comments) cover a lot of the advantages that GIMP has I would also talk about, including:
- Lenient license
- Hackable (ease of scripting, creating extensions, etc.)