I have been using the Gizmo Project client on my Mac for several weeks now and mostly I am impressed. I did get a Plantronics DSP500 USB headset which is both comfortable and clear. That made a big difference compared to using just headphones and the built-in microphone (using both built-in microphone and speakers is a bad idea – too much echo for the person on the other end of the line who gets to hear everything they say repeated back on the line).
Mostly, I have been making trans-atlantic calls with it and have found it to be very stable for a beta release. There have been a couple of updates (including one that restored the ability to record calls on the Mac). The only real problem I have had is with their ecommerce backend. It has been down a couple of times now so the client believes that you have no credit and won’t let you make call-out calls (i.e. calls to the PSTN). It also won’t let me buy $20 of credit at the moment. Couple that with the fact that during the beta you can only buy one block of time each week, and it could be annoying, but in fact the $10 lasted over two weeks.
I am also working with VoIP at work at the moment – porting the open source VoIP PBX software Asterisk to run on one of our wireless access points. You can read all about this work on the Devicescape blog, including a mini-review of a Wi-Fi VoIP handset from ZyXEL.