Already some time ago a friend of mine told me about syncthing. I've been using foldersync for longer time and was quite satisfied with it. I've been using it to sync the photos from my smartphone to my NAS. This works fine in the background as soon as the smartphone is connected to my home's wifi. That's why I have never been really tempted to switch, even after reading an article about syncthing in the c't.
Things changed when my wife got a new smartphone. I didn't want to connect i with my google account. One drawback of this decision was that I would have to buy the foldersync app anew, a matter of 5€. This reminded me of syncthing and I decided to give it a try.
While syncthing works fine when trying to sync data from the internal memory of the smartphone, things got more complicated when syncing the DCIM directory that's present on the externall storage SD card. Actually syncthing would not even report finding any file :-(
Problem and solution card be found in the support forum : syncthing tests for the presence of the directory by looking for a file .stfolder. This prevents full data deletion in case the whole directory is not present. Now with Android 4.x there is this nasty write permissions issue on external SD card, preventing syncthing from creating the .stfolder file.
Solution was simply to create the file using ADB mode, and setting up anew the shared folder. Sync started immediately afterwards.