I have used SharpDevelop to create an ASP.Net WebForms project - didn t have too many issues with it. The lack of a visual designer is certainly annoying, but it forces you to think about the source directly, which is a good thing...
I haven t done anything with MVC yet - though from the sounds of things the only thing that s stopping SD at the moment is the tooling. The core developers might have no plans for that, but it s an open source project, so there s nothing stopping an independent effort.
I ve gone a very limited amount of work (bug investigation) with the SD code - it seems to me that once you understand their classes it s not bad code... Their failing as a project has been (as it is with most open source efforts) that everyone wants to code, so the documentation gets ignored. No-one likes writing documentation, but clear documentation might have led to far greater participation...
It s a great project, but their decision to ignore the web is madness.