Already using skype on the iPhone...
Got Skype for an hour now. (have the Japanese iTunes access and the time zone allowed me to get the software a day before State side).
It's pretty much just... skype. doesn't work over 3G! i can only make calls from WiFi. for some that's enough. but still, it would be nice to have it over 3G...
the interface is beautiful. i still prefer Fring for it's multiplatform.. but Skype looks great and audio calls to other skypers are great. don't know how iPhone to iPhone would sound. haven't given that a try, but iPhone skype to land line is perfect, just like you'd expect and audio comes through clear. no delay. (WiFi of course)
it's free. so... might as well give it a try.