"During this period, services will run between New Cross Gate and Dalston Junction, Mondays - Fridays only. There will be no early morning or late night services, with a 7am - 8pm pattern (or thereabouts) running. Full evening and weekend services will begin on the official opening date."
The full service is due to open on May 23rd. Once it's fully up & running, Transport for London estimate that around 100,000 people to use the service daily. As the line offers a link between parts of London that have long needed good transport connections, it should be a success.
One of the more important questions, is posed by Londonist: "It does, however, pose a significant hazard to pub quiz-masters and London Underground trivia hounds: should London Overground be counted as a proper Tube line or not?" More crucially, will it be included in the Tube challenges, where people try to visit every station on the network in the shortest possible time?
UPDATE - looks like this soft launch might not be happening until next week now - London Reconnections