Must say I am pretty impressed with Google Talk. It took about 3 minutes to download and install the software and get going, most of which was taken up figuring out which sockets to use to plug a new Plantronics 70 headset into my notebook PC. The main drawback is that I only have one person to talk to, but it is the first day! Also, it only does VoIP with the Google Talk client software, and both of you must have Gmail and run Windows.... The IM bit, of course, uses XMPP and works with several clients.
I did a little piece for today's Guardian and a longer one on Google Talk and Google Desktop for tomorrow's Online.