I am looking for a great, as platform independant as possible, GUI toolkit.

My favorite language is C++, however; if I need to go to another language that isn't too much slower than C++, I would be willing to learn it.

Basically, what I am going to be doing is I need to create a fairly large project that relies heavily on a GUI. The main problem is that it has to be fast, it's going to need access to sockets and/or classes that can deal with e-mail, and it will need to run on Windows at first, then OS X and Linux.

What is my best option here? I don't want to go Java because it's too slow and it doesn't feel native enough. The applicaiton has to have the OS' native look. It would be great is the GUI toolkit had expandability to look nice like Office's components but it isn't necessary. I know I could use each OSes specific GUI toolkits, however; I would like to use the same source from OS to OS. I could use wxWidgets, however; the setup never seems to go smoothly for me and I always end up with compiling errors with their own samples.

Should I bite the bullet and use wxWidgets, or is there a better alternative? What do you use for cross-platform projects?