Truphone is a free app. It allows you to make calls via wifi only, so it does not work over 3G because Apple and AT&T say that's a no no. When you register on their site you get a $4.00 credit. Calling USA and Canada it's .06 cents a minute. Supposedly if you call another Truphone user the call is free. Also according to there website you can send SMS, but I havent figured out how. Not a priority for me since it's .20 cents per.
So what about voice quality? Voice quality is actually really good. Maybe even excellent, but it may depend on your wireless connection.
The application is pretty simple and is similar to the standard phone app. It integrates with the built in contacts.
Why use this app? Well it can save you money on international calls. Check Truphone - Cheap & Free International Mobile Phone Calls for rates. I use it to save up my minutes and try to build up my rollover minutes. I figure it's .06 vs .45 cents.
The one thing that doesn't work is bluetooth. In other word Truphone does not recognize any bluetooth headsets. The audio comes only thru the iPhone. That sucks. I emailed them about this and they said they may add this in the future. Let's hope soon.
I give this app 4 out 5 stars.
iCall is a competing app but has not been released yet but same premise.