Over the last year or so most people I know have moved to Skype, as it works on OSX and Windows. Also IM (and voice) conversations are encrypted, and it works through firewalls with no configuration (unlike iChat).
I've also now moved most of my phone calls to Skype, and am generally happy with call quality (as long as I'm not downloading lots of large files at the time
More and more of my friends are getting cameras too (usually as they move to Macs
) so video is becoming more normal as well; when we want it anyway.
At my last work I used MSN, and it was quite unreliable connecting and sending messages sometimes. I'm glad to have moved over...