Scripting
The absolute greatest thing about it is how easily you can integrate scripting into it. At first, I was very hesitant to try scripting for it, thinking it was going to be one of those clunky languages with a feeble link to the rest of the software. However, turns out it's very much C style, but almost object oriented, which is great! And it's an absolute dream to integrate scripts and code snippets with what you're doing. Within seconds, I was happily scripting away without any noticeable interruption to my workflow with the rest of the package. (Granted, having studied Computer Games Technology at uni probably helped!).
Putting it to use
So why am I (being a qualified games programmer) tinkering with hobbyist software? The answer: Project[N]Resource, a small hobbyist games development community which I've been a part of for a couple of years now. We've tried making games in the past (and have got one long-term project in the works just now), but we've never actually finished anything, as we've never had the tools.
The real motivation for using GameMaker? Aside from hopefully learning a lot as a community about game production, YoYoGames are running a competition (for $1000) to make a game with the GameMaker software (either with the free or commercial version). This is a great opportunity to get the guys together and make something fast (the deadline is about 2 months from now)... time to start pushing ourselves!
I can't say too much about our plans just now, but I can say that we're working on an isometric strategy kind of game. Lots of challenges to be addressed, but I've got some solid prototyping done, and am looking forward to making a playable game with the guys. The prospect of some prize-money is alluring too, of course! :)
Onward to victory!