I've been using VoIP at home for a while now, and it has been great value. A landline to mobile (cell) phone call on an Optus $49 cap for 9 minutes would cost AUD$7.55, but on my VoIP plan with www.pennytel.com it is AUD$1.00.
I found out recently that Pennytel is also a Virtual Mobile Operator, reselling Optus SIMs. I subscribed to one of these at $8 per month and set up their 'smart dials' which leverage a few tricks, and that $7.55 call cost me $0.14! I'm still amazed.
NB: the Optus call was calculated using their published rates of 0.35 flag fall plus 9 x $0.80 per minute.