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One has to distinguish between a "180 degree vee" and a "boxer" engine, as well. Both are "flat" in that the cylinders are horizontally opposed, but one has both pistons going right/left at the same time, whereas the boxer design has the pistons coming in together, and going out together, thus requiring separate crank throws for each cylinder, unlike the common vee configuration of sharing a throw for each pair of opposing cylinders. They have substantial differences in balance. The 180d Vee does, as Will states, have terrible balance characteristics. The boxer, however, has very nice primary balance, decent secondary, but a rocking couple forced by the separate crank throws, which tends to make the crank ends try to describe a circle.
I don't know of any mainstream designs that used the 180d Vee in favor of the boxer layout. Ferrari used a wide angle Vee in the early 60s on some racing engines, but the VW, Porsche, and Subaru flat fours and sixes are all boxer designs, as are the BMW (bike) and Citroen (2CV) flat twins.
A 60-90d V6 has wonderful balance characteristics, so perhaps Will is misremembering his SAE book. Not as wonderful as a boxer 6, but still very good, with better packaging. The Alfa 60d V6 is turbine smooth.
For those of you who haven't read any engine and chassis design books, reduced vibration is not just a matter of comfort. Vibration can easily destroy an engine, and too much imbalance significantly limits peak revs available (and thus peak power). An inherently well-balanced engine requires much less reinforcement in the block and less substantial bearing saddles to support the loads, which means the engine can be smaller and lighter for a given power output, or can produce higher revs (and power) for a given weight and size. If the engine itself is lighter and vibrates less, this also means the engine mounts and the engine cradle can be lighter, which creates a nice cascading effect with making everything else lighter. This improves performance in a wide variety of ways, from straight line acceleration to cornering power to fuel economy.
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