To be honest i used to use Game Maker a few years back then i couldn't code yet. But i have not used it in a long time. What i can tell you is that the learning curve with stencyl isn't that steep with and there are hundreds of examples on the net and videos to show you the ropes. After a short while developing it became extremely easy to add more content into the game without cousing too many bugs and problems with the game mechanics.
However the actual stencyl program does have one or 2 minor bugs in it. For example saving sometimes refuses to save the map? I would have to restart stencyl and do it over. That only happened once or twice. But thats not a serious issue.
I can happily recommend stencyl to anyone. Its a very powerful tool and by the looks of things, they developers have big plans with it. :-)
Don't forget to check back with the blog. I added a few updates since last;-)