Random lurker here. I've used all three programs (though I'm mainly an MMF user) so I thought I'd give you guys my two cents here:
The biggest flaw in MMF is that you can't create an engine, i.e. edit the engine, and the whole game changes accordingly. Instead you make all the code (or MMF's equivalent to code) in one level, then copy/paste it to the next, and the next, and so on.
No offense, but this is simply not true. I'm not going to go into details, but it's more than possible to keep everything in one "frame" and build level editors and scripts for content creation. And like Annabelle pointed out, it's been done before
and it's been done good.
a lot depends on the sort of game you have in mind. what sort of game do you have in mind to make?
Its a auto side-scrolling run & gun game. 2D with pixel art. The enemies will work off pre determined paths as opposed to AI. Kind of like a platformer-rail-shooter. I envision the pace of the game to be very quick, with lots of projectile dodging. Parallax Scrolling layers for the background.
For that kind of game, you'll be fine, regardless of what program you choose. Just go for the one you feel more comfortable working in.
And to address your original post:
I wouldn't go for Construct if I were you. I'm very excited about the project because it's basically a technologically superior version of MMF, with a vastly improved programming system, and without all the cumbersome legacy objects and other quirks that MMF has. But, as you mentioned, Construct is still in its infancy, and the Scirra team are still working on it. And it would suck to have your game be ruined by some stupid bug after working on it for several months. But then again, it was a while since I last used Construct so it might be more stable now than how I recall it.
As for MMF vs GM, they're both fine, you can't go wrong really. Some things to note though: GM has longer loading times, but has vastly superior 3D and vector graphics capabilities (the MMF dudes are still working on this). Neither has cross-platform support as of yet, but GM is being ported to Mac last time I checked, and a Java runtime for MMF is in the works as well. GM can be extended through the use of third-party DLLs, MMF has a similar extension system.
It really comes down to which program you're more comfortable with, and how much you're prepared to pay for it: GM costs $20, MMF is $100 for the Standard version and $300 for the Developer version (and believe me, you don't want Developer: It's the same as Standard except you get to sell your games without having to mention MMF somewhere in your game (I certainly wouldn't mind), and you get a bunch of useless extra features).
There is a free downloadable demo for each of these programs, both which are feature-complete, but don't allow you to "compile" your games. So the choice is yours. As I said, you can't go wrong, they're both great programs.
I hope this helps, good luck!