I've just had a test drive and have some comments in the hope that they mighty be useful. It's quite possible I might have overlooked things in the following. Overall I'd say I liked the idea of ZD but was disappointed at the execution.
1) Far too slow for a desktop app. At one stage the preview pane divider was lagging 3-4 seconds behind the (slowly moving) mouse. Yes I know (think!) it's XUL, but it's frustrating slow even on a decent laptop (Core 2 Duo, 2.6ghz, 4MB, Win 7 Pro, SSD) still. It really needs a (say) Qt/C++ version, or even Qt/Python version for local use.
2) I generally found lots of times where I wanted to do something but couldn't due to both functional and configuration limitations. For example:
a) turn off the auto-expansion of a clicked calendar day in month view to the hourly view. If you don't want that, it's highly annoying.
b) set the colour of entries in a specific single calendar
c) enter more things than just appointments in the calendar. What about birthdays, anniversaries, notes, general reminders, general notes (some of us record notes about what we did at various times), etc.
d) set calendar entry alarms to 12 or 24 hours, or more than 18 hours generally, before the event. Some things need very advanced reminders!
e) view more than 1 month in the calendar - why can't I choose to see as many weeks as there is space in the window?
f) set whether just IMAP headers are downloaded, and whether they are locally stored between sessions or not. I get the impression that ZD always downloads the full messages, which is not often what you want with IMAP.
g) set whether the preview pane was on or not. This just kept coming back! They are dangerous and shouldn't even default to enabled. The UI kept resetting itself generally - I got a bit fed up with continually re-expanding my IMAP folder.
h) disable mail grouping
i) Get the version number! There's a lot to be set for standard menus with a HELP/ABOUT, at least as an option.
j) get an alternate email "from" identity selection to stick as the default
k) sync the calendar with Google. I'm sure I read this was possible, but couldn't see how to do it.
4) And some weird things:
a) I couldn't get my usual IMAP server settings to work (for receiving email), it just would not connect. I entered them exactly as I always do, and have done on other working mail clients. It worked when I switched to SSL mode. Normally I don't use that.
b) Not copying sent email to configured IMAP sent folder
c) one sent email appeared in "sent" and "outbox" simultaneously for several minutes, before eventually disappearing from the outbox.
5) And some desirable omissions:
a) address book via LDAP server
b) syncing with Symbian. Still 40% of the market.
Hope that's useful to someone!