VOIP is a great functionality that allows you to make and take phone calls over WiFi. Some services are free, like Skype if you call from Skype-to-Skype, but most of them just cost less than a regular phone call. TruPhone allows you to turn your iPhone into a VOIP phone and provides you with cheaper calling fees, but it isn't quite as solid as it needs to be to make it a must have app.
If this app worked for the iPod Touch, many people would find themselves in a much more comfortable place calling wise. Granted, unless you live in a city with free WiFi everywhere, you'll still need a cell phone, but it would make for a pretty nifty tool. TruPhone requires you to register your regular phone number and email address with them before you can start. They also call to confirm whether it's really you or not. This is all because of the fact that when you call out, it shows your regular number.
As of right now, TruPhone only lets you make outgoing calls. You're not able to receive incoming calls through TruPhone, only through AT&T. This is a huge knock to the overall value of this app, especially for anyone hoping to negate overseas charges. You could always just wait for them to call, hang up, then call them back.
The rates for TruPhone aren't bad, but many people become discouraged with VOIP because they expect it to be free. TruPhone provides a pretty decent call quality for what you're paying for the app (free) and service (cheap). If you'd like to save a few bucks and have great WiFi around you, definitely check out TruPhone.