Currently, Stencyl is partially out as SlickSet, which I have messed around with a little, so I might be able to jump into Stencyl when it's complete. However, I am still completing Phoenixan in GameMaker... I'm most likely going to pick up on original projects in Stencyl, like my space exploration Metroidish game (untitled at the moment), and a spy game I was working on concepts for.. that'd basically be a 2D version of Goldeneye 64 or Perfect Dark.
But yeah, it is compatible with Mac and Linux as well.
Actually, if anyone is willing to take it on after the game is released and Stencyl is out, I can hand over all the sprites and resources to someone for Phoenixan's porting if I want to take on other projects instead.
So far, the only apparent downside though as been that in Stencyl or Slick, Phoenixan actually has higher requirements than it does in GameMaker 7.