I find myself in the nasty situation that my game tools need to support Mac OSX and I can't demand Windows only. The good news is that I don't have to port any tools -- this is a ground up write that can take both platforms into account ahead of time. I'm free to choose any language I like, but an attempt with Python and wxWidgets convinced me that it wasn't a path I want to follow. Right now I would very much like to use C# for all my tool stuff and just build it in a way that works well across Windows, OSX, and potentially Linux.

I already know that Mono's runtime works well enough for my purposes, so I'm not concerned about that. I'm also not concerned with rendering. The big question is what to do about the UI library. I want to build fairly complex interfaces, and I only want to do it once. That tends to suggest either WinForms or Gtk#, but I don't know how well WinForms works on OSX or how well Gtk# works in general. I'm hoping one of you has "in the trenches" experience on doing cross platform C# interfaces and some recommendations about how to do this.

My experience pretty much mirrors Nypyren's - WinForms isn't an option, and Gtk# is unpleasant in any context.

I'd definitely take a look at going the Python/Qt route. Qt is solid as a rock, and Python makes it vastly more bearable than in it's C++ incarnation.

If you don't have a requirement to match an existing tool's UI, how about going out in left field, and trying something like Kivy?

Unfortunately it's more the Python that I have a problem with. It's not a style of language or development that I'm particularly fond of. I may have to put up with Gtk#'s wonkiness, because I'm not sure what other options there are short of C++/wxWidgets.

because I'm not sure what other options there are short of C++/wxWidgets

That's a hell I don't envy you.

It's really a shame that Qyoto has never been fully stabilised on Windows and Mac.