I regularly use Skype to chat to friends, and the voice quality is superb considering the call is free.
I also recently did away with a landline altogether and replaced it with a Voice-over-IP service from Vonage. I still use my office telephones as though it were connected an old fashioned POTS system, but instead the calls are routed over the Internet. Calls anywhere in the UK are now free, anytime of day or night. It’s a nice touch to be able to tell customers you can call them back to save their bill knowing it won’t increase yours.Quality has ranged from superb to the occassional echo, but rarely any complaints. The bandwidth the service uses is fairly small, but I have noticed a loss of quality when I’m download huge amounts of data from the ‘net. Overall though, combined with Telewest’s 4mbit Cable Service (shortly to be increased to 10mbit!) there are no issues. Oh, and Vonage will automatically send any Voicemails you receive as .WAV files to your e-mail. Superb for when I’m on the road!