This morning I participated in a multinational conference call using Skype ... a good test to see how the technology works. Answer: it's almost there, but it's not quite ready for prime-time (or maybe we're not quite ready for it yet).
The good things: in contrast to conference calls using plain old telephones, I find setting up and instigating a skype conference call much easier. Select the menu item for start a conference call, pick the people you want to talk to, and dial. No need to remember the telephone number for the conferencing service, no need to pre-book a virtual conference room, no need to enter pass codes. You can also see who is available for the call in advance, and you can use instant messaging during the call - so no need to spell out those cumbersome URLs.
The downside: it seems not all Skype clients are equal. In this morning's call we had two Macs, two linux machines (I think), and one windows machine. The linux clients kept failing to connect. My Mac client required the occasional restart. You also really need the right gear - headsets with headphones and microphones are thoroughly recommended. You can just about make do with headphones and a laptop mic, but be prepared for a lot of noise from typing, and a deafening rumble when the laptop fan kicks in. It's all but unusable if you use laptop mic and speakers, as the feedback echoes mean you can only talk in short sentences.
If you take into account the new hardware requirements (which at £30 for a usable headset are not prohibitive), and are prepared for some weird bugs between platforms, Skype is a more than adequate alternative to traditional conferencing services. Given the price that companies like BT charge (we've spent hundreds of pounds on calls in the past), this is definitely a good thing.