I have been using Zimbra for only about a week, but I already love it. I have set up my own install and I'm the only one who uses it, so I can't really say anything about the collaboration features, but it's also great as a regular mail client. I actually got it running on a PIII 700 MhZ with 256 MB Ram, also it's a bit slow sometimes. The same machine also hosts a few PHP sites using Apache and those have seen a small slowdown since I installed Zimbra, but they are still running fast enough for me (I don't have many visitors anyway...). I plan to write about the tweaks I've done to get it running acceptably fast on such low-end hardware and when I do so, I will post a link here. I have got only one request: Make Zimbra faster on fewer ram (or do a single user version or something like that ).
Update: here is the link: http://the-wizzard.de/2007/05/20/zimbra-on-256-mb-ram/