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.