• What it does?

It just connects to your Nightscout server and provide support for other applications.

• Does it have GUI?

No GUI suitable for regular users at the moment. But maybe someone write it.

• How to use it?

Write your own app and listen to broadcasts to get new data from Nightscout. Send broadcasts to write to mongo database.

• Any other features there?

NS Client can emulate xDrip app if you don't have physical device available. It supports xDrip extension for DanaAps too (OpenAPS implementation for DanaR pump)

• Some examples?

Look at the and of MainActivity

• And how do I get response?

When you upload treatment you should receieve it back if you listen to treatment broadcasts after it goes through mongo

• Do I need care about success/failure of upload?

No. NS client takes care about it. Undelivered records are queued and delivered when client reconnects to server.