I hate them. Never been a fan of them. Its something about talking to a disembodied voice that makes it hard for me. I pick up _alot_ from visual clues when talking that helps me process what they say in the proper context of what they actually mean...you know, mood, attitude, gut feeling...that sorta thing. But on a phone, I just can't do it. It makes me feel...self conscious.
So I prefer email or face-to-face. Mostly email when it comes to business because I can think out my reply (you know, let the emotion go) and have a record of it.
Outside of business relationships I prefer face-to-face because that is when my personality really comes through. It is hard to make sarcasm work in email.