Hey luxgud
The GDK is a games-engine, but it's much more than Irrlicht, Ogre, ect, in the sense that those forementioned engines are really just rendering engines, whereas Dark GDK handles rendering, plus everything else you'd expect from a professional engine, such as collision, multiplayer, sound/music, file handling, and input (to name a few).
It can't (as yet) be used with anything but VC++ 2008
A few years back I was about to buy the A6 version of the 3D Games Studio, but instead bought DBPro, and yes it wasn't easy at first, because I'd had absolutely no experience of programming before, but with a little help from the forums, reading up on reference books and using a bit of common sense (I want to make a cube -> "Make Object Cube"), I got into it and found it the easiest language I've ever used so far. Using the Dark GDK is essentially no different, you just need a basic amount of experience/knowledge of C++, and as 3DGS uses a C syntax scripting approach, I doubt that's of any concern.
As a bottom line, Dark GDK can potentially do anything that the A7 engine can do, and more. Because the GDK is in a C++ environment, you can use just about any library you come across and integrate them together, and with the available library's direct from TGC (such as Dark Physics and Dark AI) you've got a great amount of power and you still end up spending less money that if you went for 3DGS.
It's free for non-commercial use, any commercial projects require you to part with cash for a licence, but as you'd (hopefully) have already created the game ready for a commercial market by the time you'd need it, that won't matter much.
As the Dark GDK is very much like DBPro, it's quite easy to use a lot of the DBPro tutorials and apply them to the GDK with a little tweaking. Besides the GDK comes with a few tutorials to get you started anyway, and there's also the forums, which have always been and always will be my favourite and best source of programming knowledge and help.
To sum it up; Dark GDK is pretty godly, and is incredibly easy and helpful for beginners and experienced programmers alike.
Hope that helped a little...
TZ