Well VoIP is my business so I guess I believe in it
I first wrote a strategy paper on VoIP (for BT) in 1994 when it was a working but poor technology. Now that we have standards based technology (SIP, QoS, etc) and widespead access to broadband it's time has come.
Pretty much every call you make today uses VoIP at some point (because it's a more economic transport mechanism than circuit switching) and by the turn of this Century BT will have converted ALL of it's networks over to VoIP - they call it the 21 Century project.
VoIP is cheap, flexible and does a huge amount of stuff that a traditional network literally can't do.
There's a small introduction here:
http://www.voipfone.co.uk/FAQ.php
Just on a couple of points of information, you can phone any other phone in the world with VoIP but if the other phone is also VoIP it can be a free call.
Skype is a proprietary network - good for kids and free calls to your relatives but not a real business service. In the next year google, yahoo, Aol, microsoft etc etc will all launch or re-launch 'Skype-alikes'
Reliability - The technology is stable, we depend utterly on it ourselves but it is not a '5 nines' industry yet. ie 99.999% reliable - which is the traditional telecoms claim. You will get the occasional glitch but in return you get amazing features and functionality at virtually no cost.