We have had voip and adsl2 for at least a year now with iinet and have had no problems. Our internet is $50/mth and all calls are free, except for mobiles, overseas and 13 numbers which are at a good rate. Our bill is now around 60-70/mth which is a lot less than we used to pay.
You have to buy a modem and voip phone (don't get the iinet phone - its junk... actually they have bob now, maybe thats better). I have no idea how much it costs these days but I think it was about $200 for us - if you have the knowledge you could of course shop around.
Your phone plugs into your modem - instead of the wall (freaks visitors out because the dial tone is different). Other than that it is exactly the same as using a normal phone.
I'm trying to think of what else to tell you. can't think of anything. Its been a good saving.