To me, this is a very welcome change, and one that should have been here long ago.
Most software I know uses docks or MDI to place elements related to the software into a single window. What the GIMP did instead, was create at least three windows belonging to the one piece software. Which gave me a lot of trouble:
* Task switching became tedious, especially when having multiple images open, because every single window showed up in the task switcher.
* Moving the GIMP to a different desktop was tedious, because the tool windows would still be on the other desktop.
* I could not simply maximize the image window, as fairly big parts of the image would hide behind the toolbar windows. Not only that, but the menu would hide behind the toolbar windows.
If they went even just MDI, two of these problems would disappear, as all the windows related to the program would then exist inside a single window. They went with docks instead, which are pretty much the same story and also gets rid of the third problem, so I'm glad they finally did this.
This was a bigger problem when they spawned multiple windows, because said windows tended to be always on the foreground, which made them hide big parts of the image window. At least now you can make the dock so small that it's not visible at all, even in fullscreen mode. This means that you can properly show/hide the tools when you need them, as shown in these pics:forces the toolbox to remain on top even in full screen image editing mode (very big mistake)
Also, the GIMP doesn't force you to use the new window mode if you don't like it. The default mode is not single window mode. Keep using the much more convoluted multiple windows version, if you like.
It does still need some improvements, though. It would be nice if you could add tabs and determine which tools/sub windows go in there, and be able to close some of the tabs that are there now (like the bottom-right pencils/patterns/gradients docks). Also, perhaps, easily folding/unfolding the docks so they're completely hidden. This is possible now by dragging the dock's border, but then you'd have to resize it back to something good once you want it back.
Still, this is a good step in the right direction if you ask me.