I send all of my friends links to Daley's guide, so of course my first suggestion is to listen to Daley's suggestions :)
That said, I'm using my iPhone on Airvoice with no issues. I can use all the iPhone features like FaceTime, Messages, etc. I only use my phone in the US for about 5 weeks a year and I don't know that I'd use Airvoice full time but so far I've been happy with it.