[Moppie]

The upsides are light weight, and reduced complexity. They are used extensivly in the avation industry for these reasons.

The downsides?
They require huge amounts of cool air flowing around them, which means locating them in areas of high air flow, which means compromises in chassis design. You also have to create the airflow when the car isn't moving, which means useing very large fans which are noisy and again cause design compromises because of thier size.

You will notice that as soon as Porsche went to water cooled engines in the 911 they also moved the engine forward so its now over the rear axle, rather than behind it. Its not something they could have done that with an air cooled engine, as there would not have been enough air flow to cool the sort of heat the HP they now produce generates.