Roger, as Daniel mentioned Gizmo on 770 has been reviewed by some users months ago.
One major difficulty on 770, which prevented popularity of such applications, was the location of microphone and lack of support for a wired or wireless headset (still I'm at loss with Nokia promoting chat and Internet calling without providing adequate hardware/driver support).
Nonetheless, I'm one of those, who favor Gizmo (or any other SIP based one) vs. Skype; on an open platform, shouldn't we go for open standards too?