I'm looking at a solution for a small company which needs to restructure its IT. Zimbra is my preferred solution for them for many reasons (one of them is your Desktop solution).
But I have to urge you to begin prioritising connectors. Your competitor Open-Xchange seems to have some fairly decent SyncML. But similarly to Zimbra there is no decent Thunderbird plugin for contacts and calendars.
Funambol on the other hand has amazing sync capacity - just tried it out with my phone and synced it with thunderbird in minutes. Very nice looking Add-On too.
What's a user to do? I can sync mobile to thunderbird but neither to Zimbra. I can sync mobile to Open-Xchange but OX is an otherwise inferior product.
My bottom-line...This is where the users are - get busy with connectors and you'll take huge market-shares. People want sync that works and all you have to do ;-) is recycle code...
Make a functioning thunderbird connector (if only contacts to begin with) and put more resources into SyncML...In the meanwhile, if your SyncML is slow...Work with funambol to get a connector working for their servers.
I really like what you're doing and this email is not meant as criticism but as incentive...I've spent 2 weeks researching solutions for SOHOs in the groupware field and there's nothing that does the job.
Zimbra is close - get there!