After my positive experience with GIMP 2.6 I've been using GIMP more lately to work on my personal photos. Last time I commented on the interface differences between GIMP and Photoshop, but the interface is nothing compared to the real strength of any graphic program: the algorithms that run behind the scenes and tell your CPU how to interpret the graphical data.
As I mentioned last time, I got the impression that GIMP handled low-light digital grain better than Photoshop. That's a pretty huge claim as grain is one of THE most considered factors in the professional photography world. Grain can ruin your chances of having a photo published, and while the photographers have gradually become more tolerant of film grain, digital grain is still hated and feared. Grain occurs in low-light conditions, but photograph-worthy moments don't always occur in brightly lit studios. So if an algorithm exists to reduce grain that is more powerful than Adobe's algorithm, it's a big deal.
One powerful tool that Photoshop has that I've yet to see matched is unsharp mask. GIMP has an unsharp mask but it doesn't have nearly the range and flexibility of Photoshop's. I have used unsharp mask to make blurry photos look sharp, to hide flaws inside subjects while sharpening their outlines, and to reduce grain. It's a very powerful tool once you learn how to use its settings. GIMP's unsharp mask is a very simple sharpen tool with a low range of uses.
The thing that really got me excited about GIMP was its filters, and here is where its open-source heritage really shines. In Photoshop you have a certain set of stock filters like emboss, oil painting, trace edges, pencil sketch etc. They're boring. They're tacky. They're overdone, and you've probably seen them a million times. Aside from 2 or 3 with more flexible uses once you tweak them enough I've never seen a more useless dropdown menu. Sometimes you can use them in interesting ways but believe me, it's not easy. They also haven't changed in the past billion versions of Photoshop that have been released. Why are they still there? I don't know either.
There are lots of third party software called plugins that work with Photoshop to give you a whole new set of filters. They're usually commercial (I don't know of any free ones), and they work with varying degrees of speed, quality and crashiness. From my limited experiences with them I've found them to be a pain.
It's always exciting to try new filters out and see what they are going to do to your image, knowing that you will have to try them in many situations to really get a feel for how the filter works. Gradually you begin to associate a random name in a menu with a set of possibilities for your image. GIMP is exciting just in the fact that it has more filters than those horrible Photoshop stock ones. It does take the few good ones that have the most wide-ranging uses and duplicate their effects under similar names. But it also has a range of exciting new possibilities, especially the fractal rendering filters. These provide a huge array of possibilities for mathematically working with your images in ways that make Photoshop cry. Some of the best art I've seen made with the GIMP uses these fractal filters. I love how many of the filters come with little explanations from their makers, telling you what they're supposed to do and throwing in some humour as well. One filter is described as "special effects that nobody understands". Amen to that. Designers usually use the effects without having any idea what they actually do to the photo algorithmically, only caring about how the end result looks. It's amusing to know that the people coding the filters haven't got a clue either, and just do what looks good.
GIMP has its bases covered with its filters, allowing you to edit your images in so many different ways. I'm sure there's a nearly infinite amount of other things that can be done but the GIMP filters sure feel more comprehensive than Photoshop's.
It's worth noting that the super-common effects: levels, curves, saturation, contrast, etc. are totally equal in quality, interface and speed. They've probably become standard since they appear in any number of other generic photomanipulation programs as well.
Back to the interface comparisons: It seems that GIMP is optimized for working with smaller-sized photos such as those suitable for web display, like a 400x600 px, 72 dpi photograph. A few complications come up when you begin working with a 2000x3000 px, high resolution still. All of the brushes are measured in pixel size and they don't come very big. So if you want to darken, lighten or erase part of your big image by hand using a brush, it's out of the question. Of course I did learn how to make new brushes of a custom size, but why can't there be a simple slider or number box to punch in the pixel size you want? I accidentally made a 1000 pixel brush and crashed GIMP. It was very exciting.
Adobe has a system of displaying big images starting at partial resolution and gradually scaling up to full-res. This allows you to see your image even if it's blurry until the full-res image loads. Handy if you're just tweaking colours or things where you don't really need to see details. GIMP does a line-by-line image read where you can watch your updates get rendered on the image from left to right, top to bottom like a progress bar. A pain if you're trying to edit something that occurs at the bottom of the image, but still better than not seeing anything until the whole thing is done. GIMP generally applies edits to big images more slowly than Photoshop, but 2.6 is a world of improvement over whatever old version I was using before that took ages to render the smallest changes.
I think this speaks to the software's main audiences. Adobe caters to professionals who often work with images big enough to print in a magazine, poster or even a billboard. GIMP caters to the internet generation, those who wish to create art and post it on their webpage or blog. In order for GIMP to move into the professional market, I think it will need to tackle the way it handles large, high-resolution images.
Because of its awesome filters, GIMP still wins.