Skype's voice quality is uneven. When I call a landline, the other person often says he can't hear me well or that my voice is breaking up or that there is a terrible echo. Even when I call Skype to Skype, sometimes I hear the other person's voice in chops. This does not happen on a landline to landline phone call. So Skype is NOT better than a landline, but in most cases the voice quality is OK enough so that I use it because the calls are free, especially to people who live abroad.

The voice quality issue is political/traditional, not technical. All VoIP vendors except Skype are sticking with 8-bit/8 KHz audio because that is what the PSTN uses. They could easily drop in wideband codecs, but they just won't.

In general the SIP vendors are building "the PSTN, only cheaper", while Skype is using their imagination. But Skype's innovations are easy enough to copy, if you have the will to do it.

The difference between Client-Server and Peer-to-Peer is that Peer-to-peer leverages the power of computers that
1)are not tied to static IP addresses / URLs, and
2)that are only intermittently available.

As long as skype supernodes meet those two qualifications (pretty sure they do) then skype is peer-to-peer, not client-server.

Pretty much every peer-to-peer network in existence uses supernodes. According to your definition this makes them all "client-server". Which would make the term "peer-to-peer" pretty useless.

When Skype says that their architecture reduces deployment costs or scalability, then I do not have an argument. But here they claim that this architecture allows them to realize high voice quality. From that perspective, I was suggesting that there is no difference between these two architectures. So how do they derive the claimed quality?

You're right that the P2P architecture has nothing to do with voice quality. Skype's better quality is just accidental.

I wouldn't say it's accidental, but I would agree it's got nothing to do with P2P and everything to do with codec selection.

As far I have seen the only unique and interesting thing about Skype is the use of the wideband codec. I'd love to be able to use wideband iLBC on my Asterisk system, pricipally as a means of connecting to ITSPs. Of course they would have to support that codec as well. From what I've read online only the standard version of iLBC is open sourced. The widebandwidth implementation licensed in another manner.