This integrated fantastically well with my corporate Exchange account; now I can use all my favorite Thunderbird addons and have the space saving ergonomic layout of Tb, plus ease of folder traversing "Next Unread Email" and tabbed mail; I don't even have to be on VPN anymore to manage my mail.
Connecting to the EWS server on Thunderbird Release was an absolute breeze, the price is really good and definitely worth the added comfort and stability. As a developer I can appreciate that this is actually not an easy task to pull off, and Kent even offered some support for a Calendar provider.
In terms of productivity, this is a huge win over the free browser access, the money spent on this was basically made back on in time on the first day.
Now if we could just have another addon that connects the Thunderbird UI to the server side filters (Microsoft calls them "message rules") ! :-)