NRefactory in SharpDevelop 2.2.1 covers whole C# 2.0, in SharpDevelop 3.0 it covers C# 3.0.
MonoDevelop uses a modified version of SharpDevelop (they're fixing bugs on their own instead of coordinating with us), no difference in the functionality.
NRefactory is just a C# parser; for code completion, you also need a type system. In SharpDevelop, this is available in ICSharpCode.SharpDevelop.Dom. The source code download of SharpDevelop contains a project "samples/CSharpCodeCompletion" that demonstrates how to use ICSharpCode.SharpDevelop.Dom (the sample also uses ICSharpCode.TextEditor, but ICSharpCode.SharpDevelop.Dom is independent of the text editor used).
I don't if the type system in MonoDevelop is available in a separate library or if it's tied to the IDE and its project system.