As so many, I tried Skype for Mac OS X and tried to compare it to iChat AV which has been a quite impressive experience so far. But I must say that Skype has been at least impressive if not more.
The sound quality of that little net telephony application is gorgeous and the echo cancellation works so good that I can use the built-in microphone of my laptop without any hassle. The user interface is as simple as it can be (although there are a couple of small things that can be improved: it‘s much more windows-ish when it comes to drag & drap).
But i should mention problems as well: in one out of two connections there has been noisy hisses and other sound artefacts that might stem from bugs in the beta version or from the protocol itself. Difficult to tell and it will be interesting to follow the upcoming development. If they manage to stomp out these remaining issues they have a winner product in their hands that will give SIP-based telephony a hard time (although I don‘t think that‘s stoppable in any way).
Another advantage of Skype is it‘s disadvantage as well: using Skype you don‘t lock yourself out of any of your other communication systems. It‘s an extension. But of course you lock yourself into Skype‘s system once you think about relying on that service (which might be tempting when adoption rate grows). As long as Skype is a proprietary and undisclosed protocol it won‘t be able to penetrate open source markets but it might get hold of a huge market share in general like ICQ did back then.
So if you like to go for a test or wanted to talked to me anyway you can look me up in Skype‘s phone book and give me a call if you like.