Maybe it's jsut me but the slots were basically useless for me. nice for fun but money wise, just (auto) clicking go me the needed money way faster and without risk of losing a lot inbetween
Yeah I'll up the slot a bit, but with the proper upgrades you can still earn quite a lot from it. It's a complementary way of earning idle money when you have auto-spin on. (It won't beat auto-clicking though!)
