Hi toby,
I have used VOIP for sometime and think it is fantastic, I have a dedicated phone server in my place (asterisk/freePBX) with about 20 extensions on it all over the world so friends, family, family of friends etc etc can all call each other for free anytime.
The only fault I've been able to find is some of the hardware phones are a little temperamental, I got 5 grandstream gxp2000 phone a short while ago and 2 of them crash very often. although the one that do work are great and I've heard other brands are more stable.