Based on my questioning a very small sample of 2 ardent iPhone users (my cousin and my father), the new early-release fring VoIP client is looking like a major winner. I had a play with it yesterday, and the interface looks impressive, and the voice quality & ease of use on SIP seemed pretty good, although my dad was having glitches getting his SkypeOut working on it.
The downside is that at the moment it's only working on jail-broken iPhones, but given the apparent fervour among Apple-istas for it, I'd hope that Cupertino sees sense and makes sure it gets certified as an official app through its developer programme ASAP.
Apart from a very early Truphone demo last year, this appears to be the first major VoIP release for the iPhone. Will be interesting to see what the other players - Truphone, Skype, the enterprise players like Cisco & Divitas etc - do in response.