I used to have the ability to inspect shaders in Visual Studio 2012, but now I've lost it. The UI doesn't actually crash, but the modeless popup message saying "This may take a few minutes" never bears fruit. I found a post at another site claiming that installing VS2012 Update 3 fixed the problem, but that hasn't helped in my case.
Back when using Pix was an option, I found that rendering outside of a BeginPaint() / EndPaint() pair for speed would cause debugging issues, but again that hasn't been the issue this time.
More info: There is a warning message initially, "Debugging information for 'YourApp.exe' cannot be found or does not match. No native symbols in symbol file". This is as I'd expect, as the main app is managed, with the DirectX work being handled via an unmanaged engine DLL that loads slightly later (once I know what bit version to use).