Ask HN: anyone using VoIP as a cell phone replacement?

I would like to be free of a cell phone plan. I spend most of my time -- and make most of my phone calls -- within range of a broadband wireless network. I would ideally like to make most of my calls over VoIP, with the option to fall back on a cell network using pre-paid minutes when necessary. With so many new devices supporting both Wi-Fi and a cell network, it seems like it should trivial to do this, but I can't seem to find any good information about it. Most of what I see are older posts that say: "it's not quite ready yet. This will happen soon." Has anyone successfully ditched their cell phone plan in favor of VoIP? If so, what devices work well for it? What obstacles or annoyances are there to overcome? If not, why not? What problems remain unsolved?