I swear that I thought I replied to this thread????
Well, in case I replied in my dreams, I have tried Joyn and had it on the stock, but rooted ICS. It was pretty cool when it worked. Too many times I had to force stop it from the settings because with Samsung's RAM and CPU manager, the app would just restart whenever I tried killing it.
That was the biggest downside to that app; the bugs. In my house I get 1 bar of signal and too many calls drop. Joyn allowed me to turn everything off except WIFI and talk for hours. I would talk to my girlfriend all day(like 6+ hour long phone calls) with Joyn without any problems or call drops. Now the bugs were related to whenever I disconnected from WIFI or had to restart my router. Once it lost connection for whatever reason, it would have problems reconnecting after WIFI connection was established. What that did was that whenever I had a call, no menu for answering would pop up(literally nothing pops up, no dialer etc). You just hear the ringtone playing and then seconds later a message saying missed call. Then comes another problem related to that. Since the phone dialer didn't work properly while Joyn was running in the background, with no connection, you couldn't make calls. You got this error message telling you to connect to a WIFI/network despite you already being connected.
It got pretty tiresome having to go into settings, apps, Joyn, and then force closing it. Whenever I used Samsung's RAM and CPU manager, the app had a high chance of restarting whenever I killed it/cleared ram. Sometimes it would kill it and allow me to relaunch it or use the stock dialer. Most of the times though it would just restart with the same connection problem.
I am currently not using it as I am on another ROM. It was really buggy on BB7. The app constantly drops calls on WIFI, the sound quality is very poor now and it still has the above problems. I uninstalled it.
Do not listen to the what people say about battery life. It greatly extends your phone's battery when calling through wifi. Just make sure to have DATA and Bluetooth off and brightness down low and you can talk for hours without killing your battery as fast as with data. I literally had 5 to 6 hour long calls with only 20-30% battery loss. Try doing that with regular data and your talk time goes to 2-3 hours maybe 4.