For my part, I'm testing the 5 RC1, and the MSN gateway is working (but still have some bugs with oftenly disconnection, compared to my old ejabberd +pyMSNt)
I must admit that zimbra is a great software, I really like it a lot. But there are some things to be done IM-client side to be really good

There are some time bugs while writing to a contact then it is sending in another window, being invisible to other server jabber user (like gmail) etc etc.
Since at the moment, it lacks stability client-side, I want to use a client like psi or gajim. But I cannot connect to the jabber server from the outside. Do any of you know how to do this ? I think it is about the configuration of the server (Wildfire ?), but I didn't find where to do this. Any tips ?