Source: http://feeds.gawker.com/~r/gizmodo/full/~3/wtUTrjKN5x0/hands+on-with-jajahs-ipod-touch-voip-app
Previously, the JaJah app for the iPhone was used as a middleman to connect people on VoIP to your cellphone, but it still required a mobile plan, and used minutes. Now, the Jajah app works entirely over wi-fi, meaning you can use the iPod touch, and make calls without a mobile plan. It also works on the iPhone, but won't work over 3G or EDGE, because then AT&T wouldn't make any money.
The strength of this Jajah app is simplicity, and that is a good thing. It doesn't try to work with a million different communication services (such as Fring or IM+), and it maintains much of Mobile OS X's design DNA. This makes the app extremely easy to use.
Placing a call is as simple as selecting a contact from your list, or bringing up the dialpad and entering a number. Similarly, sending a text over IP (handy for international SMS) looks like it's pretty much the same as sending a normal SMS, except that you're inside the JaJah app. I got to see the UI for sending an SMS, but because the service isn't set up yet, I didnt get to see it go through.
As for call quality, voices sounds a tad on the tinny side, but not to the extent that I would say it's bad quality. There's also a few seconds of lag while the data passes through JaJah's servers, yet that's no huge surprise either.
The cool thing is that JaJah would like to work with Mobile Carriers on branded solutions, and expect carriers to offer flat rate VoIP service using ! the JaJa h technology. This means you wouldn't need to buy minutes, you'd just pay one dollar amount every month for unlimited use. In any case, VoIP junkies should look at JaJah when it comes out in a few weeks.