Yeh I had it on my iphone-its OK but you have to have push notifications enabled to use it and it drains my battery-have a iphone 3gs. I used it a few times over wifi and on 3g was a bit hit and miss-I would use it on holiday-lets say im overseas and want to talk to my daughter (she has a iphone) then free to chat but just make sure you have a wifi connection-call clarity was crystal clear-not sure how many friends you have with iphones but only have maybe 4 so I just stuck with my normal minutes on o2.
It's an absolutely fantastic App .
I'm now regularly in contact via mobile with my Mum and my sister in the UK from here in Australia .
In the past , calling a landline and they weren't at home often meant a half a day delay because of the timezone - now contact is instantaneous
Text and Photos are lightening fast - and even sending video is free .