I've used Zarafa for a customer and they are pleased as punch. However, there are some short comings:
No Spell check
No "Next" or "Previous" links in the message pane.
Signature doesn't work consistently
It will work with Outlook, after a fashion, but only for three accounts with the Open product. If you need a local copy of Outlook for more users, you'll have to go with the Commercial version.
That said, it works well - I will be switching to it for my in house messaging solution once I flush out the mobile part (Thanks!!! Darrell May!). But for a group of users that are used to using Outlook locally, if your don't want to use one of the many connectors, you're stuck with Exchange.
If you have users that are used to a webmail product, then Zarafa for a calendar solution, or roundcube without a calendar are both excellent choices.
1&1 has a nice webmail product that I've implemented for a few customers, and it runs less than $5. (US) a month per user.