I may as well throw in my totally uninformed voip opinion too!
Does voip work? Yes, subject to you using the right kit, same as anything.
Does it need to cost a bundle? No, not if you think about it.
Will it go wrong? No, not if you have thought about it. Use reliable kit, have spare capacity etc and you should never have a problem.
It is invaluable in some situations, I run a helpdesk service, my people all work from home, so its fantastic for us. It (asterisk currently) sits on our vpn/IM server, and gives us the ability to put phones on our clients sites etc.
The right kit? Dedicated server, with a hot failover in a separate datacenter. Hardware phones, QoS on our broadband. Business grade voip->pots service (Gradwell).
Shame we will never get broadband in the Uk anything like our buddies in Norway or Sweden. Even Norway, where the beer is very far from free, dedicated 50mb duplex fiber, installed 500, and 50 a month. Japan, 100mb duplex for about 60-100 a month.
So its perfectly doable, workable etc, you just have to watch out for the cowboys. As usual.
As for call charges, I'm not 100% how much cheaper than BT it is, I can not for the life of me figure out their calling rates from their website, but I am pretty sure they bill by the minute. Gradwell offer lower call setup costs, then bill me by the second at what I'm pretty sure is substantially less also. End result, much cheaper, more functionality and flexibility.
Its a tool, and like any, will not work for every job. To select the tool you need, remove preconceptions, research the subject, plan your implementation etc.
Also, avoid reading the comments on sites like this. (Even mine :-) For example, the thought of everything in my business grinding to a halt because of a switch is hilarious. I have spares. So should you, or a service contract that is quick and efficient.
Paris, because I'll bet even she wouldn't swallow some of the reasons punted above.