It probably all depends what line rental costs you are paying. When I transferred to OPTUS broadband last year I bundled my phone with it and because I had an ABN I got it cheaper again and it works out to approx $18.00 / month now. Combine this cost with VOIP charges ($10.00 / month + $0.10 / call) I am getting away with about $35.00 / month . This was previously $100.00 plus.
I kept my landline as VOIP doesn't recognise some numbers such as the local pizza shop etc that have numbers starting with 13...... and for when the power goes out as well. You do get another number with your VOIP but you still retain your original and if the VOIP is hooked up correctly it will all ring on the same phone.
I personally don't find the call quality to be as good but for the price I am happy
Cheers
Rod