Finally I've managed to set it up and take it for a spin...so here's my 2cts worth

Zimbra is nice... very nice...

The Tags, Conversation groupings, seaching and sorting functions are great...and highly useable thru the web ui. Faster than most web ui available...on my test envioronment.

1) Attachment rendering...
-I'm not sure of the rationale for this, but finds it buggy, a number of Word docs and PDFs could not be rendered...its nice when it did work.
-but it gets annoying when I cannot download the actual attachment I need ... eg a zip attachment. It seems like once it has been rendered, it cannot be downloaded in its original form...for editing...

2) HSM... This is nice, perhaps additional rules can be set in future to move messages to slower storage based on both size and age

Haven't tried the Clustering or Backup/Restore... but the overall user experience is very good...