It's easy to deploy, scales well, is easy to support, and all of the hard configuration work is already done by the install script! I've tried multiple times to do a "from-the-ground-up" install with a similar configuration, and it's always been a ton of headaches, and a lot of trial, error, and reading manuals from dusk until dawn, and back to dusk again.
I run a small mail server with 4 virtual domains, iRedMail, phpList with a promotional mailing list, and I've customized it by using OpenDKIM instead of Amavis for my DKIM signatures, but other than that, it's pretty stock, except for Apache virtual hosts, and rbl blacklist checking that I've added. It works beautifully. I couldn't ask for more.
There were other options out there like Citadel, but I hated the interface, and the general configuration, and wanted a true Postfix / Dovecot setup with a minimal amount of headaches. That's where iRedMail comes into the picture. The fact that it's based on open-source packages, and free software is amazing. I can accomplish in 15 minutes, what it used to take a whole day or more to do manually. Even then, I'd have to do extensive testing, and configuration changes before deployment.
While I still end up doing configuration changes, and tweaking settings before deployment, I feel a bit safer knowing that I haven't overlooked something in a configuration setting for postfix or dovecot that leaves me with a gaping security hole, open relay issues, or a ton of backscatter to deal with. iRedMail has been absolutely wonderful for me. It's been rock-solid, and reliable, and the community / forums have been great as well. I always end up getting the answers that I'm looking for in the forums. I really appreciate this community, and iRedMail in general!