Lately I have been using MSN Messenger for voice and a bit of video conferencing. However, to get this to work I have to reconfigure the NAT services on my ADSL router to a much less secure configuration (unfortunate lack uPnP implementation on the ADSL router). Messenger traffic is not secured either, so it was time to reconsider alternatives.
So I am back with Skype. Only just. As soon as I installed it on my new AMD Athlon 64 box, it reliably crashed immediately on startup with some sort of memory access violation reported in the Windows event log. As pointed out by this article, the reason turned out to be an aggressive data execution prevention (DEP) configuration. Turning DEP off for Skype solved the problem. Tomorrow I'll find out whether it really works across my symmetric NAT configuration.
Update [17/05/2005]: Unlike MSN Messenger Skype seems pretty happy with my NAT configuration. After some initial problems with my contact list it now works reliably and is likely to stay as my voice client.