Has anybody ever installed "Altezza" brake lights on their [DSM]?

Legality varies from state to state, as well as country to country. You would need to check with local law enforcement or motor vehicles department to determine if the "Altezza" lamps are legal for use in your area.

At the very least, any aftermarket lamps installed within the USA should be Department of Transportation (DOT) certified. Many also have SAE (Society of Automotive Engineers) certification. Very generally speaking, if a lamp is SAE/DOT certified, it may be legal for use in your area, but this is no guarantee.

Many "Altezza" styles are illegal. Some are missing side marker functionality that is required for OEM lamps. Also, many 'clear' tail lamps lack red reflectors and appropriate side markers, making both styles less than fully compliant with state or federal laws. 

In recent years, many manufacturers have been forced to recall "Altezza" and clear tail lamps for offering lamps that violate federal or state standards. Affected makes include APC, CK Motoring, JKL Auto Accessories, OBX, Pilot Automotive and Robocar. Verficiation of these recalls is easily available from the NHTSA's recall database.

Those interested in a more thorough perusal of automotive lighting standards may find the privately-run Motor Vehicle Lighting Standards page a good place to start.

Legal questions aside, many DSMers have installed Altezza brake lights on their cars. 2G owners have several vendors selling "Altezza" style lamps. 1G brake lamp configuration does not appear to lend itself to the "Altezza" style, but a few vendors have nevertheless managed to create them for 1Gers as well.Autosupermart.com did offer them for the 1G Eclipse at one point. And if all else fails, there's always eBay.

Many Altezza users complain that the aftermarket lamps are not sealed properly, and therefore leak or collect water.