is SDK worth getting at the moment and why?
Yes, if you just plan to just play with it and learn how it works, no if you plan to use it in a real project. Cos its broken.When the promised update appears, (Promised for last week), then it would be worth useing in a project.
Its a little tiny tiny bit slower than B3D itself, (from dll calls), but the surrounding languages (c#,c++ and BMax for exapmle), more than make up for this by being faster. Unless you use Pyhthon, or the like, which are slower.
could i use it with C# express edition.
Yes