Anonibet used to have a 10% deposit bonus scheme for it's sportsbook that could put you at a mathematical advantage over the house, however they stopped it once someone figured out how to exploit it and started telling other people.

For the most part though deposit bonuses are designed in a way that in order to clear your rollover requirements you pretty much always have to lose your initial deposit and deposit some more. That said, if you're just playing for fun and want the BTC you deposit to last a bit longer then they're great.