Ideally, I'd want something more than syntax highlighting. Something that gave me all the features of Visual Basic itself:
- Select a keyword or object reference and press F1 to open the online help for that keyword.
- Automatic fill-in and description of parameters for functions.
- Debugger with breakpoints.
(etc.)

I've just never tried opening up Visual Studio and trying to see if one of its output types is "VBscript". And now I'm in a position where I'm trying to choose whether to go through the hassle of installing Visual Studio on my devbox here at work just to find out the answer to that question.