You shouldn't have any reservations about sticking with the base 2.5-liter four-cylinder engine that Mazda6 'i' (including base SV, Sport, and Touring) models get. It's reasonably fuel-efficient (not nearly as much as class leaders like the Hyundai Sonata though), and it feels perky whether you go with the six-speed manual or five-speed automatic.
Step up to the V-6, however, and you might not be able to go back; the 3.7-liter engine churns out 272 hp, along with plenty of accessible low-rpm torque, making the Mazda6 feel like a muscle-sedan. It's a hoot to drive, but it's also quite a bit thirstier. No matter what, you get nice capable brakes with a firm pedal feel, reasonably good, athletic handling, and a ride that straddles the line pretty well between comfort and sharpness.
Those expecting a serious sport sedan, or moving up from a Mazda3, might want things even tighter though; the current 6—especially in V-6 form—can feel a little too heavy and isolated.