I have been using the beta for ages now, but the final release does appear to have some new bits in it, so check out this page for a list of features.
I can't wait to try out the new video and audio structure...hopefully it wil be better than it has been. The Messenger backend has always "just" handled audio and video and never really done it well. They may have got it right this time as Microsoft have been doing a lot of work on the enterprise and business side of things with the as yet to released Communicator product.
The other interesting feature is the tight integration with MSN Search...the shared search feature is an interesting one...nice touch..and i'm starting to see its uses after only playing with it for a few hours.
Well done guys...
PS. I'll be doing a marketing/communications blog post about the new messenger over at the Principius blog later today.