From an email I received from them:
If you sign up with a referral you receive a free share. They say Apple, GoPro, Dropbox or others as an example but I wouldn't get my hopes up for an Apple share.As of 5 April BUX is offering 2.01% annual interest on your uninvested cash balance.
That's right - you can start earning interest compensation by simply depositing funds into your BUX account. And the best part? No other Dutch neo-broker offers a higher rate
Your deposit with BUX is held at ABN Amro Clearing Bank and is protected up to €100,000 under the conditions of the Dutch Deposit Guarantee Scheme (DGS).
Interest compensation will be paid quarterly on uninvested cash balances up to €25,000.