I've been using Gizmo for a few weeks now on my E61, but only for unimportant cheap calls. Like calling a friend to say im there to pick him/her up, or asking what time for something.
The thing is, Gizmo sounds like crap even if you use their PC software. It's not the phone.
I dont know why so many people got WiFi problems, and why they need a VOiP frequency for their wireless routers. I got a cheap cheap D-Link DI-524 and I didnt even need to open special ports. The SIP client connects to the internet, not a frequency.
I can even get it to work over GPRS. There is a 1 second delay between the time I speak and the time the other person hears me. But there is a 0.2 second delay for me to hear the other person speaking. So, for 1.1 cents a minute, it's not so bad.
Though Id suggest you guys upgrade the firmware. When I had the original firmware of E61, it was impossible to hear each others over GPRS, and sometimes even WiFi. Now it's decent with the new firmware, there is no skipping or distortion, so you can talk on it, though still low quality.
I did try VoIPtalk, but aside from the echo test, I couldnt do anything else to test. The quality was much better on the echo test, so Id say its possible to get good VoIP on the E61, even on GPRS, as long as you find a good service provider.