I'm not particularly a fan of stuff that requires a compile step -- I'm way too much of an instant gratification junky. Doing iterative design and development of dynamic UIs leads you toward an addiction to seeing your changes right now. That seems to me to be one of the biggest advantages of doing your UI right there on the client, or using a scripting language on the server -- you can make major changes to your Web app super-fast to respond to customer needs.
But haXe does seem like a nice way to leverage the power of having a One True Way in building your Web apps -- in the flavor of Open Laszlo or GWT, or Phobos, only with better coverage, because it's got all of three: JS, Flash, and server. (Of course GWT and Phobos don't make you learn a new language.)
The haXe compiler is GPL, and the libraries are BSD. The Neko runtime is LGPL.