libsecondlife is an excellent library for letting your application interact with OpenSim and Second Life grids. It has been designed for a non-gui application so don't confuse it with the OpenViewer project or Linden Lab's own viewer. If you want yet another GUI 3d viewer then use another project. But if you want to do amazing things without having to grok the monstrosity that is the full blown viewer, this is the library for you.
Maybe I am wrong but from a glance this project is reinventing the wheel just so it can be written in .NET.
That doesn't sound very liberating to me and others who don't use Microsoft Windows. I suggest a name change.
Also the secondlife viewer has been released as GPL 2 (I realise this was not the case when the project started) and runs on Linux which makes the official project more valuable then this for the simple fact that its goal is to run on more platforms whereas this projects goal is to run on just one.