Very reliable.
The Mazda 3(Protege) and Mazda MX-5(Miata) are Mazda's most reliable cars.
These days, all newer cars made after 2000, even Fords, are made to last 10 years or 200,000 miles so you don't need to worry much with any newer car.
The Mazda will definitely last a long time. Maintenance is expensive at the Mazda dealer. But everywhere else, it is cheap. It is not as expensive as German cars to maintain so you should be fine with that too.