If anyone has any familiarity with Gamemaker, then you may be happy to know that version 7.0 is out. Also, development has been taken over by YoYoGames - hopefully with the result that there will be faster dev time and more resources devoted to the community.
We're going to use Gamemaker in the Game Design class I'm attending, so I went ahead and bought the Professional version (it's only 20 bucks). The Pro version basically makes certain things much easier - including a built-in particle system.
The community thus far seems a little more engaged than the one for Blitz3d (largely because the B3ders seemed to move over to BlitzMax, the Mac dev software). Though the comparison isn't quite fair since these are two very different programs.
Gamemaker's strengths are in the top-down and side-scroller realm. 3-d is possible, but you'll have to program a lot of functionality yourself and are better off with Darkbasic or Torque. The learning curve is pretty small, though I can't really recommend the tutorials on Gamemaker's own site - they, to put it mildly, suck.
I would stick with the help file and the Game Maker Community messageboard.
I've also ordered two of the books on the market specifically geared toward Gamemaker: The Game Maker's Apprentice and Basic Game Design & Creation for Fun & Learning. I may put up evaluations when those arrive.
As a test I sketched out a rough block, scanned it in, did some editing in GIMP and then exported to .gif. After that it took only a few button presses to get my hand-drawn art into a room. I threw in a snowman as a player, gave him movement functionality and tested the game. Voila! Big ugly penciled blocks and a snowman. Pretty cool.
The hardest thing is scaling everything properly and selecting the borders so that the player surface doesn't look awkward. That and trying to properly animate anything when your experience is close to zero.
Still, it's a great introduction. Some of the games coming out are great throwbacks and it's a great opportunity to learn the entire game development process on a small scale. It also gives me a greater appreciation for the complexity of those old school games, especially when I consider that they didn't have access to a handy little tool like Gamemaker.
When I finish something I'll be sure to post it.