asterisk users - Mar 2008 - To what degree can Asterisk replace Cisco Unity?

In a CallManager environment (currently 4.0, moving to 6.1 in the next
few months), can Asterisk completely replace Unity
as a voicemail system?

What works and what doesn't?   MWI?  Call Handlers?  Does everything
work via a SIP trunk? Who has done this
and is willing to contact me?

Thanks.