Update 11-09-09: Having a 2-month experience, I still like the service. With near proximity of the new competitor Ribbit, Google Voice should be more proactive to capture the bigger market.

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I just got an invitation for Google Voice today, so I had an opportunity to test in at the different modes. And I like it very much. Ability to perform conference calls is just a single fracture of the full package you get.

Making conference calls is easy. You give your Google Voice numbers to all the callers and invite them for certain time. Every one, calling at the time of another (main) call is in process, can be connected to it. This way, the call initiator can include all the callers in the conversation.

The service is still by invitation only, but you can order your invitation from the link.