Ive just managed to download it on my PC and copied it to my iBook G4.
Installation is quite simple, there is a big Microsoft ('wizard' dare I say it) that will setup your .Net account etc. One step in this installation is checking for previous installations. This took fooorrreeeveer, well seemed to anyway so I just skipped it.
Loaded it up and the contact list looks pretty good. Its still nothing on iChat and the actual list is very similar if not identical to 4.0.1. Your display picture is at the top next to your name. There are now two tabs, coprate and personal aswell so you can switch between coporate and personal accounts.
The conversation window has hardly changed (identical to 4.0.1 aside from the brushed metal look). The display pictures 'drawer' was a novelty at first but within 2 minutes I have closed it in order to slim down the chunky window. One note about this display pic drawer, it is grey, not brushed metal like the rest of the window.
Having no idea about the Corporate side of messanger I can't really say how good it is nor can I say I am bothered it was included.
So apart from a brushed metal theme and pictures that I found oversized and irritating this version 5 of messenger seems to me like a pretty poor update. Look at it this way, take version 4.0.1 paint it silver and you have version 5, well almost. What has been done will definatly make messenger more pleasent to use, but its still far behind MSN Messenger for Windows.