This is the first stable release in the 2.5 line. Yes you could use the new UI before now with pre-releases, but that's been true for a long long time.

I am not sure if this means the python API is stable, too.

Quote:

    For the next 2.58 release the API will be completed
    with access to internal notifiers and events to track
    data changes or UI refreshes.

I thought 2.6 was suppose to be the stable release.