bought this last time around, have been using since September and LOVE it. i'm from teh technology dark ages, so this was a huge leap for us. i previously had a conventional land line with the local phone company and DSL internet, was paying close to $100/mo. i've since signed up for mediacomm cable tv and internet, bought the Ooma, and ditched the landline entirely.
for about the same monthly expense i now have great cable tv (never had this before), awesome internet speeds, and no phone bill. Ooma bills me less than $4 per mo for the local taxes, and the call quality is excellent. the only problem is if you have a power outage or interruption to your internet, you have no phone. i am considering buying a cheapie pay as you go cell phone to use as a backup (we don't own cellphones otherwise).
i ported my old phone number to the ooma for i think $30 so i didnt have to change numbers. this took only a few days. you can sign up for Ooma Premier which gives you additional calling features and services, but it's nothing i personally need. this is their main revenue stream i think - other than the initial purchase price of the units. you can also purchase Ooma handsets, but mine works perfectly with the cordless handsets i already owned.
WELL worth the cost. online chat-style customer support has been speedy and spot on. unit very easy to install and operate. i can check my voice mail from anywhere i can access the web.