We tested wm6 quite extensively on different devices, via gprs/hsdpa and wifi. No problems at all..
Initial sync can take a while but not more than about 10 minutes for a full sync (over wifi).
Big mailboxes, sync of many folders and settings on device to download complete attachments and/or large parts of messages, and/or any of these combined can bring you a lot of data-traffic and thus bottlenecks on device, memoryspace, connection etc.. (and corresponding expenses..)
Check these basic settings and see if you can optimize your sync-cycles..
We are using Zimbra mobile in production and migrating blackberry-users who are not complaining (yet..)
(not been able to test the brandnew blackberry connector for now..)
New users are happy with zimbra-mobile, ms direct-'push' (..) and mail and calendar synced over the air.
Using mainly HTC-Kaiser and HP514 devices.
Maybe this helps
Using ZCS Network-edition 5.0.16 on Ubuntu 6.06.2 LTS and 8.04 LTS