Rated 4 out of 5 stars

Mostly works, but Owl for Exchange does not handle well being regularly logged out (presumably due to my organization's aggressive security policy).

Problems:
1. I regularly have to click through a cumbersome log in process. After being logged out, Owl for Exchange presents a token challenge screen in a Thunderbird tab. If this form sits too long and times out, then to re-establish a connection I must (1) enter a purposefully wrong token code, (2) acknowledge the login failure, (3) log in with username/password, (4) enter token code. If I enter a correct token code at step 1, then Owl for Exchange does not correctly re-establish the connection and I must quit & restart Thunderbird to make it work again.
2. I have lost draft messages several times. If you try to send a message after a connection was lost, it can be lost if you quit and restart Thunderbird. Be sure to copy/paste unsent messages to a text editor before quitting Thunderbird.
3. I regularly have to restart Thunderbird to have Owl re-establish its connection.

Make sure you use Owl for Exchange for a while before paying; some problems only appear after extended use. There is no support.

This review is for a previous version of the add-on (1.2.4).