This version includes mostly bug fixes, in particular fixes for some small memory leaks.
The only new feature is 574273 Z-order management of edges, which adds support (via new contextual menu entries) for controlling which edges are considered "below" or "above" which other edges. The effect is mainly visible when combined with the "Jump Links" feature, in which case it allows for controlling which edge "jumps" above the other.
The update site are available at http://download.eclipse.org/sirius/updates/releases/6.6.0