I've been using it for little over a year (for the same reason as you - didn't want to put too much info into Google), and would describe it as half-decent. I use it as a diary, essentially - schedule, calender, todo list, etc. I still use Gmail for actual email though, because unless you own the domain outright (i.e. not a subdomain) you can trigger false positives for spam on other servers that results in you mail being dropped.
-pain to setup - the target audience is sysadmins, so you really need to practice doing it in a VM first (especially since doing a complete uninstall is non-trivial)
-buggy - every time I've upgraded to a new version, I've run into a significant bug. (Inability to handle apostrophes in names, buggy apache module that keeps crashing the server, etc.) You can usually fix it if you know PHP/Python, but it's still a pain that makes for a poor user experience. (I've submitted bug reports with patches for these, but not all of them have been merged.)
Basically, it does take a lot of effort to get it working (I have 500 lines of notes on it), but once you've done that it will keep working pretty reliably. The official client is KDE Kontact, which works great for me since I use Linux anyway. I can see it working out poorly if your OS is Windows though (KDE for Windows isn't stable yet). It syncs well with Android too. I generally prefer to enter important events, etc. in through the webui though, since I don't quite trust the other clients to sync them properly.
tldr; it's probably the most broken piece of software I've used under Linux, but it works well enough that I continue to do so. I wouldn't recommend it for anything important (e.g. actual business use), but it's fine for non-critical personal use.