We just did a cost evaluation and feature comparison at the end of the year. (doing this from memory)
We looked at the feature set of exchange/zimbra and renewal costs per year including costs for the advanced pack CALs for exchange (or whatever they call it). At any rate, costs rolled out 4-5 years still seemed to benefit zimbra, particularly if you figure that you have a server license + CALS on top of the Exchange License + CALS. Also, to get the same (or similar) feature set from Exchange that you get with Zimbra, you have to evaluate the enterprise version, which really made it cost prohibitive....
We are a relatively small shop with 3 windows boxes and 4 Linux boxes, and the Linux boxes hands down are cheaper to administer IMO. I also liked the integrated spamassassin/clamav, since that was how I had configured the previous sendmail setup.
At any rate, we went Zimbra (obviously), and though I just turned things on live yesterday and am short on sleep, so far, am very glad we went this route + it was fun as heck beating up on the box while users loaded 100 GB of email from all over the US and things kept plugging away as if nothing was happening.
But to each his own. Both are stellar products. Go with what you feel comfortable with.
1+ Year on Zimbra
Release 6.0.6_GA_2330.RHEL5_64_20100505193959 RHEL5_64 NETWORK edition.
RHEL 5.3 - After complete and utter Ubuntu 8.0.4 failure.
AMD Dual Core 2216 x2
16GB of RAM
425 GB (6 disks) of RAID 10 storage
3ware + Seagate SATAs
Virtual Server as emergency system using rsync.