Syncthing is awesome, I don't understand why some people still use bittorrent sync. If you use ownCloud only for file syncing you'd also be better off using Syncthing instead. The developer is also a super nice guy, answering all bug and performance reports on github.
Regarding contact & calendar syncing, I just wish more apps could just use files instead of always setting a new server and reinventing the wheel to sync data. I still have not found a good way to export my calcurse calendar to my Android phone via files only.
Calendar and Contact syncing should be pretty much standardised (for open source things) around CalDav[1] and CardDav[2]
I'm not quite sure what you'd gain by doing this process manually (export on one device, import on the other) rather than just using a {cal/card}dav server?
There are many reasons I prefer Bittorrent Sync over Syncthing: identity based sharing (rather than node linking), good support for situations where you cannot poke holes in a firewall, selective sync support, and encrypted read-only secrets.
Moreover, it's much easier to set up, I share stuff with family and they didn't have problems setting it up.
Note that Syncthing just put out relay support, so you don't have to poke holes in firewall if you don't want to, and your data is still secure, as the relay server just relays the encrypted stream. In the (roughly) week since the feature was released, the public relay network has already routed over 2.6 TiB. (I run 2 relay servers on the public pool)
Relay support is nice, but I still desperately wish for "caching relay servers". I'd love to be able to spin up a high bandwidth VPS with storage and let it be a node, but for it never to have the ability to actually decrypt the data it's storing.
Both BitTorrent Sync and Syncthing need a particular port in the firewall to be open (or port forwarded). I'm a contractor that's almost always on the client site, and corporate firewalls are not opening up because I'd like it so. I've been using ownCloud for the past year and since it just uses the https/443 port, it works everywhere.
Exactly why ownCloud implements CalDAV and CardDAV, the common standard for Calendar and Contact sync. No reinventing the wheel here. Also, exporting your data is as simple as pressing the download button and getting an ics / vcf file.
Regarding contact & calendar syncing, I just wish more apps could just use files instead of always setting a new server and reinventing the wheel to sync data. I still have not found a good way to export my calcurse calendar to my Android phone via files only.