@SamuraiCrow:
MUI Builder has been bugfixed by Mazze, an AROS developer. I've got commit privilages to MUI Builder also. Do you have any bugs to report? (Do you have any tips for MUI programming? I could use those too.)
Sorry, Samurai, but I wasn't clear. I've only lightly tried MUI Builder, I was simply suggesting to JJ that the closest he was going to come to what he wanted by using it.
Also, the other project I was refering to that "got eaten by a grue" was Ganymede, so I was refering to something "close to Visual Studio" as JJ was inquiring.
Finally, as I mentioned, I have lightly tried MUI Builder on Icaros and its bugs are glaring. Just try it. You can't miss it.

(And if you do miss it, let me know and I'll send you a screen shot.)
@ ChaosLord:
I think you want PowerWindows on Aminet. It writes normal AmigaOS GUI code, not MUI.
Got a link for that? I went looking for it and the only thing I found was a tool that would allow me to move a window off the screen. I don't think that's the same thing.

@ JJ
Actually, thinking about it further, if you want a tool that comes close to Visual Studio, you might wanna try... Visual Studio.

I do some of my Amiga tinkering on VS. While I don't get the visual interface for building a GUI, I do get the syntax highlighting, method tracking, refactoring and module location that you get with VS. Once again, in combination with MUI Builder, that might get you somewhere.