I am working with both CadLib from WoutWare and RealDWG. CadLib is the best non-Autodesk .NET library for AutoCAD. CadLib is pure .NET multi-threaded that beats over super heavy single-threaded RealDWG library. However, CadLib is still missing some minor features from official RealDWG. But so far, it is a very good commercial library.
RealDWG is a real pain to work on the web comparing to AutoCAD .NET API that works only on a single AutoCAD application.
Teigha.NET from Open Design Alliance is very similar to RealDWG, I can say almost 100% code in RealDWG can be used in Teigha.NET. This library is a wrapper of C++ code like ObjectARX.
CAD Import .NET from CadSoftTools is another good library but it does not support to save DWG format, just only read DWG.