For Mac systems with a built-in microphone, Gmail calling "just works." You can talk through the mic and get reasonable quality without feedback from your built-in speakers; better yet, plug in a pair of headphones and you can use the computer's mic with no risk of audio overload. Steve Sande, who was my iMac lab rat this morning, found that he could open a new browser window and make a phone call, all without any additional set-up.
Users on mic-free systems like the Mac mini may need to make audio preference adjustments to get the ball rolling. Be aware that you cannot change these settings mid-call; they will not be picked up by the Google Talk plugin that handles audio conversations. Best to sort out your microphone before initiating your call.
There are several ways to tweak your audio settings. You can launch System Preferences, open the Sound settings panel, select the Input pane and choose your mic. Alternatively, option-click the Volume slider at the top-right of your Finder window and choose your input device that way.
The microphone I use with my Mac mini is part of a headphone set. Its acoustical pickup properties are designed for use while the headphone is being worn. If you need to wear your headset, you may want to set your audio output accordingly, so you're not trying to listen through a pair of inert headphones through to your Mac's speaker system.
Using my headset this way really made me miss the simplicity of Sande's built-in iMac microphone. I have a Dell Mini with a built-in mic that I'll be testing later today, and I suspect it will work as beautifully as the iMac solution.
Also worth noting:
if you have a Mac less than 2 years old, and a pair of iPhone headphones with the integrated mic, you can use those headphones with your Mac and choose 'External Microphone' as your input.
The greatest drawback to using the browser-based Google Voice service was lag. Sande and I experienced nearly a second and a half of delay, causing us to trip over each other's words -- a common artifact of VoIP services. I generally do not experience this problem when using Google Voice with a normal landline or cell phone.
You can easily measure lag by slowly counting from one to twenty at a steady pace. Instruct the person at the other end to match your count, as he or she hears it. The lag corresponds to the delay between when you say your number, and when you hear the other party say the corresponding number.
For anyone with a built-in microphone and work to get done, Mac-based Gmail calls promise to be incredibly convenient. Yes, there are lag issues, but free coast-to-coast phone calls with clickable calling (and no holding a phone crooked in your neck) introduces a new level of utility to Mac users.
If you want to supercharge your browser calling, there are handy extensions for both Chrome and Firefox to allow one-click dialing of phone numbers you see on web pages. Unfortunately there's no corresponding extension for Safari... yet.