This release is aimed primarily at developers rather than for production sites, because its planned lifetime is just six months. If you are building a site for a client you would likely do better to use Joomla 2.5 for now, and wait for the release of Joomla 3.5, which will be the stable long term release, and which is due in another year's time. However if you are interested in the future of Joomla then this is it.
We plan to support Joomla 3.0 with all our extensions that currenty support Joomla 2.5.
For extensions that have been written natively for Joomla 2.5 these may well work in Joomla 3.0 without modification. Our extensions that were originally written for Joomla 1.5 may require some slight modification to make them compatible. In the coming weeks we will be testing these and making Joomla 3.0 compatible versions available.