The 15% is only for 2 years. After that you'll have to pay 25% corporate tax.
Dividends are on top of that. Capital gains tax is 19%-26%. Social security, etc. is relatively high as well.
Spain is a great place to live but you don't go there to save on taxes. You have to go there with the mindset that you'll spend a lot on taxes but you'll get your money worth (great roads, healthcare, free schools, decent air quality, etc.)
Whether you get your money worth or you can get the same things in return for free or cheaper somewhere else is a matter of personal preference.
I think Spain makes little sense if you're focusing on earning money, but can be a good place to retire or semi-retire.