Nearly every app I develop now requires it to be always available i.e. online & offline capability. It can even do Peer-2-Peer (device to device) sync which is mind blowing. I like how its 100% open source too.
I’ve also tried Firebase powered by Google. Its fast ,easy and free , BUT so was Facebook’s Parse, which is shutting down at the end of 2016. So running a business critical app on it is not a good idea.
“Realm is an open source database”
Some parts are open source but the core part of Realm is proprietary.