Super happy to see Symbols taking shape :)
I've had a quick play, and found some of the behaviour a little unintuitive (maybe because so used to Illustrator). Main things which were unexpected are...
1. Creating a symbol doesn't convert the select object itself into a symbol
2. Selecting multiple objects > Create Symbol doesn't create one combined symbol from them, instead it creates a symbol for each object. In order to create a symbol from multiple objects, they have to be grouped first.
See here: http://recordit.co/CUqRsFBG9t
3. When nesting symbols, creating an instance of a symbol, which exists as a child symbol of a parent symbol, causes the any nested occurrences to disappear
See here:http://recordit.co/bIKBn0Yp3f
Hope this is helpful. Keep up the good work!