We launched two weeks ago (March 1, 2011) but our community has been extremely active right from the start. We got a number of things right (thanks to all the support and advice we received from Lithium on best practices), including having all the recommended promotional plans in place, a good board structure, a nice and clean UI and thorough internal communication around guidelines. This has resulted in a smooth launch, with about 150 new registrations and 25,000 page views every day.
From launch we also immediately had support from numerous knowledgeable brand advocates (from third party navigation sites) who have been picking up just about every question posted on the community.
We thought it would take months before we had a vibrant community with active brand advocates answering all the questions - but it all came together on Day two! Using the Engagement Center ROI tally (tweaked with our own cost-per-incident), in our first two weeks we handled 20,000 cases and delivered a value of 150,000 USD!!