Can I use leaded race fuel or aviation gas in my car? Is it worth it?

You can use leaded fuel, with the following caveats: 
-  leaded fuel will quickly destroy the catalytic converter. 
-  leaded fuel will damage and eventually destroy the oxygen sensor.

Leaded gasoline also leave white deposits on the O2 sensor and housing; some mechanics may recognize this as being from leaded gas, and deny your warranty for a new oxygen sensor.

Some people also believe that aviation gas is dirtier than normal pump gas.  Opinion on this is divided, with some people saying it is and others saying it isn't. Max Burke reports that aviation gas has none of the detergents usually present in automotive gas. This could be called a 'purer' fuel, but since it can't clean the engine as well, the engine might get dirtier inside as a result.

The most popular AVgas is 100LL (100 octane low-lead mix), which nevertheless still contains at least as much, and possibly more, lead as normal leaded automotive gasoline.  The "low-lead" rating is in comparison to other aviation fuels; this amount of lead WILL wreck the above mentioned auto components.  One Digest member suggested adding the Alcor TCP additive to the avgas to help prevent lead buildup in the engine.  Consistent use of leaded gas or AVgas will probably lead to significant clogging of spark plugs and oxygen sensors.

Except for the above details, it appears that for practical purposes AVgas can be considered the same as normal leaded automotive gasoline. According to the Gasoline FAQ (alternate link here), even 100LL has more octane than most auto gasolines, so you will get the same performance benefits as with unleaded race gas, as described above.  Also, some people can get aviation gasoline much cheaper than typical race gas prices, making the prospect more attractive. For more information, read the very detailed Gasoline FAQ (alternate link here), and this essay on high-performance gasolines.

For those who can't afford "straight" race gas, try using the gasoline blending chart provided by Osborn's Auto. Be certain to read thearticle that accompanies it.