[Review of 45.0.1161, Jan 3 2017] I have been using this add-on for over two years, and also use DavMail side by side for IMAP access (in order to delete attachments). It works well, for the most part, and I will continue to use it. I cannot complain about the license requirement, as I assume the developer needs to eat, and $10/year is really, really not that expensive.
My primary complaint regards its inability to update and/or compact folders without basically locking up Thunderbird. I often wondered if this happened with Outlook (hard to believe it would) and I don't see it with DavMail. This I/O process could use improvement - either make it happen in the background or change it in some other way in order to reduce the effects on the client. This problem only affects me every couple of days, however, and I've learned to work elsewhere for 5min while it does its thing.
[Review of 52.3.1200, Aug 8 2018] Hey so I'm back, I'm still using ExQuilla and my review is up to 5 stars. Not sure if you improved disk I/O or not but it doesn't seem to lag like it used to, and most importantly it keeps me from having to use any other email client (especially Outlook). I have read that you're not able to continue maintaining and I'm sorry to hear that, I for one would gladly pay more for the license ('cause you have to eat, too), but perhaps not many others would. Regardless, I so greatly appreciate your work and wish you the best.
This review is for a previous version of the add-on (45.0.1161).