We migrated 2500-some users months ago, and we're happy, but I'm getting a fair amount of pushback from people who want their old IMP webmail back, because if all you want is email, it's faster than Zimbra. Some of the things mentioned at
Looking for ZWC "cruise mode" productivity are addressed in 5.0, but most aren't.
One of our students rants about this at
Life, Carleton, and all things Bill
On the plus side, Zimbra actually has a good IMAP server (except for the *huge* move bug in 5.0RC1), so if you really wanted to point IMP or Squirrelmail at it, you could. The IMAP servers that ship with the other finalists in our evaluation, Exchange and GroupWise, are much more badly broken.