From another site:
Inline Engine:
Uses vertically mounted pistons most of whose vibrations must be balanced out with the use of sizable crankshaft counterweights or a balancer shaft, adding to the weight of the engine and robbing it of horsepower. Vibrations that cannot be dampened produce unnecessary wear in engine components.
Inline systems tend to have shorter axle lengths since the transfer must be located offside of center. This results in greater amounts of disturbance reaching the vehicle frame, thereby affecting vehicle ride quality.
V Style Engine:
Allows the fit of a larger number of cylinders in a smaller space but they suffer from vibration as well. This also means the addition of a balancer shaft, adding to the weight of the engine and robbing it of horsepower. Like the inline, V style engines also have higher profile than that of the boxer resulting in a higher center of gravity. In addition, higher placement in the vehicle compartment can also cause the engine to pierce the cabin in the event of a front-end impact.
The design of both Inline and V style engines mean that the crankshaft is only supported by relatively small main bearing caps.
Boxer Engine: (Subaru WRX STi)
Provides better balance because the movement of each piston is countered by a corresponding piston in the other bank. This diametrically opposed action allows for a more efficient use of power without extensive loss to vibration, resulting in reduced engine wear and increased overall reliability.
This design inherently offers improved throttle and torque characteristics and superior vehicle handling due its lower center of mass and positioning as close to the front wheels as possible. Short stroke, large bore aluminum engine also offers lower weight and size to provide better fuel efficiency and overall performance at higner RPMs.
The transmission and transfer can be located along the centerline allowing maximum axle length. This means the suspension can more easily travel over dips and bumps with fewer disturbances reaching the occupants insuring a smoother more comfortable ride.