Rated 4 out of 5 stars

Excellent tool.
Worked so far nicely for all components (address book, calendar, tasks).
However, rare corruption of address book happened (but probably due to conflicts with a different version on a Windows account).
After clean up no more issues.

NOTE: Current nightly build (http://www.gargan.org/en/Mozilla_Extensions/SyncKolab) is compatible with Thunderbird 5.0.

This review is for a previous version of the add-on (1.5.1).