CardDAV-Sync FAQ
From dmfswiki
Please append your question to the end of this page.
Is CardDAV-Sync Open-Source Software?
Currently it isn't, but it is planned to make it Open-Source once it evolves to version 1.0.
Why don't you make it Open-Source right now?
Because it's not ready yet. Some parts of the source code are quite messy, it would be too embarrassing to publish it ;-)
Is it safe to use two-way-sync?
I'm using two-way-sync myself and I know of many users using it as well. So far I've not heard of any serious data loss or corruption. Still, the source code it not stabilized yet and every modification can bring new bugs, so use it with care. (Making backups on a regular base is a good idea anyway).