In a blog post about SharpDevelop, Tim Anderson writes that SharpDevelop contains a built-in editor for WiX.

The SharpDevelop GUI in the background and a sample of an executing MSI on top.

But after having a closer look, I was somewhat disappointed:

• SharpDevelop contains indeed a template for a creating a setup with WiX, but that template is far from complete. There are no "Select Feature", "Modify", "Repair" or "Uninstall" dialogs forseen. Only the absolute minimum for getting an application installed is prepared.
• No autocomplete feature for the WiX source code (i.e. "Intellisense" in Microsoft jargon).
• No nice GUI, you still need to edit the XML source code.

On the positive note:

• It's a good starting point, but you'll soon extend this template to create your own.
• It contains custom bitmaps. This was one of my own concerns: I'm a lousy graphics artist but I didn't want to rip (which would probably be illegal) graphics from other MSI's (or MSI tool builders, like InstallShield and Wise Solutions).

Below you can find some screenshots from a MSI that was created from the SharpDevelop Setup template: