Who is the target user for GIMP?
Professionals are almost all heavily invested in Adobe and it's interface is much too hard for a casual user?
One of us (the windows user) has a copy of CS6 which if needs be he can use to make any minor amends before exporting out PNGs etc. If a designer has forgotten to export out a certain layer on it's own too - that sorta thing. For everything that _we_ need to do - (eg cropping out, making sprites, minor color amends, some blending) it is absolutely perfect and just as good as Adobes offering in that regard.
I would go so far as to say the selection tools are actually _much better_ than in photoshop. I find selection transformation a lot more intuitive and easier to do in Gimp.
As an added bonus we've saved ourselves at least £3,000 at last count on license fees. I have also found as a side affect that because we can't open the PSDs there and then (usually), we're more likely to get totally complete assets from the design team that we can get to work with. Beforehand we'd just get dumped on with several PSDs as is.
We have applied a similar workflow when it comes to office documents too - making heavy use of Libreoffice for all but a few users that really do need to edit and send out docx "exactly".
Finally, for what it's worth I have never seen the big deal about it's UI either. It follows the same sort of paradigms as many graphics tools so I've never saw that as a barrier to use. I appreciate you aren't suggesting this at all, I always saw the "Gimp is for amateurs or is rubbish compared to Photoshop" argument as arrogance or snobbery. Especially in office environments - I think (some) people associate the cost of the PS license with a validation of their skill.
I can't imagine any designer I know joining a design agency that didn't provide professional tools, which in today's world means a fast Mac, a big cinema display and the Adobe CS.
The developers are using GIMP to do simple cropping and amendments to prepared images. GIMP works just as well as Photoshop for this simple use case. In fact, the post says that they find it better than Photoshop in some regards. Furthermore, some of the developers are using Linux, on which Photoshop is not available.
So, given that they're using a tool that works for them, that they like, and that runs on their choice of operating system, what's insane about that?
From what the post implies, the designers are still using Photoshop. They're the ones who need it - or think they need it - or are used to it and happy and productive with it, which is perhaps the most important consideration.
I don't want to just hate on open-source graphics software - a lot of amazing work has gone into GIMP, it had fun things like content-aware resizing before Photoshop did, but Adobe has 1000X the resources to invest in usability and they've used those resources well.
Also, there are things like creating seamless textures, tiling them that is easier in GIMP. Animated gif import / export is pretty easy to do too. If only GIMP could do smart object work and live layer styling, I'd be all over it. But I do use it as a professional designer / developer for specific use cases. Another designer and I had a discussion. Basically since CS2, there hasn't been a valid reason to upgrade Photoshop really other than gimmicky stuff.
I'm a fan of saving money wherever we can. For what its worth I've lost track of how much we've spent on our photographers studio/rig/software. We're def. not adverse to spending money on software here and all the designers get a crispy fresh creative suite when it comes out.
As linux, foss friendly devs though GIMP fits perfectly into our workflows and in 3 years of "we no longer need to use photoshop" we've had minimal problems with it. For those instances we really do need to consult and work off the PSD our one copy suffices to do what we need before getting back into GIMP.
But more generally - a lot of people I know which are starting out with game programming work with The Gimp. Also pretty much anyone who wants to learn serious image manipulation, but not willing to either make illegal copies or spend hundreds of dollars on such an application (don't know, maybe that's not much money for you, but for most people it is a lot). And certainly people on Linux where PS simply isn't available.
Casual users are no longer the target.
At any rate, there are multiple free software packages that can do the "creating art" job at least as acceptably as GIMP. Consider MyPaint or Krita.
He has a number of tutorials available on his site too: http://www.davidrevoy.com/4-tutorials.html
I believe he also uses Gimp with the Gimp Paint Studio presets for some things: http://code.google.com/p/gps-gimp-paint-studio/
Though like so often: Only because a software wants to support a specific target doesn't mean it will be completely unusable for doing everything else.
The Gimp definitely doesn't meet the needs of a professional, but it is still very powerful. If you have some reasonable knowledge of how to use it or Photoshop you can do some amazing things. I think it does a very good job of meeting the graphics needs of the opensource community and people who need the power of an editor like Photoshop, but don't need all of the new advanced features of Photoshop and don't want to be a pirate.
Personally I love the Gimp now that I am used to it. Even when I had Photoshop (acquired through my school) I found myself using the gimp instead. I just wish it was more polished, the fact that brushes are still so clumsily implemented is really frustrating. Oh well, if I really want to paint something I will use MyPaint, which is an amazing project that has developed a million times faster than the Gimp.
In Windows GIMP refuses to show up in front of other windows. I must minimise everything else until it shows up. Single window mode works perfectly.
It also helps motivate us that the vast majority of free image editors on OSX are infinitely inferior to e.g. Paint.NET.
I've been using Linux since 96, or so, and GIMP is about the only image editor I've ever used.