This is looking like yet another reason to leave iOS permanently for Android.
I have my own domain(s) and my own Zimbra server with full calendar functionality and integration. In the past, I had (Mountain Lion) iCal setup to connect to and sync with my Zimbra server, and my iPhone configured to sync with iCal. Not necessarily ideal, but it worked and I got my calendar items on all my devices quickly and easily.
I use my MacBook Pro and multiple Windows machines (both work and personal machines) so my primary mail/calendar client is Thuderbird/Lightning. Since I have TB w/ Lightning installed and configured on all my machines, and I had iCal set to sync with my Zimbra server, and my iPhone syncing with iCal, I was always able to create calendar events on one machine/device and have them show up everywhere automatically (except for the phone, which would show up the next time I synced with with my MBP).
I upgraded my MacBook Pro recently to Mavericks and, out of the blue, no more iCal (replaced by Calendar) and no more MBP Zimbra calendar to iPhone sync. The calendar item is still in the Calendar app, and I can still create calendar items with it, and it still automatically retrieves calendar items created from other devices/clients, and they still show up properly labeled/organized. But, nope, they won't sync to my iPhone the same way they always did.
Meanwhile, I also have two Nexus devices. My work phone is a Nexus 4 and I have a Nexus 10. They don't natively support iCal/CalDAV sync with their calendar app, but I was able to download an app that provides Zimbra CalDAV support. No computer sync, no separate computer app to mobile device sync, the Nexus devices just sync directly with my Zimbra server; something the iPhone can't do. So, I figured I'd just take that same step with the iPhone and download an app that allowed me to sync my Zimbra calendar directly to my iPhone. Nope. No such app exists.
Once again, Apple has made a deliberate decision to push users away from their products. I've been an iPhone user since the first generation iPhone and, for the first time ever, I can honestly say, my iPhone 5 will probably be the last iPhone I ever buy.
Hey Apple: What's the opposite of innovation? I would really like to know what you call your new business model.