For devs wishing to test this gem with an external NTLM server (i.e. Windows 2012 Server), I've written up an NTLM Integration Test Plan that will get you started.
I was trying to use Savon pointed at a Sharepoint intranet site guarded by NTLM authentication. The ntlm experimental branch of httpi didn't actually work for me and seems to have gone somewhat stale.
So I switched the ntlm implementation to use rubyntlm which is native ruby, works on all platforms, and supports NTLM v2.