In 88, they were called Galant Sigmas (mine has Galant on the rear mudflaps) and was badged as a Galant with the Greek Sigma symbol in the back.
In 89-91 they changed the name to just Sigma, so as to not get confused with the newer 6G Galant that was being sold at the same time.
They came with the SOHC 6G72 12 valve V6 engine, which is the same motor that is in the base 3000GTs.
They are actually quite pleasant to drive, and came with a lot of advanced options, like adustable power steering (normal, or sport mode for tighter steering feedback) and height adjustable suspension. Mine also has the automatic digital climate control.
The other thing that set it apart was the climate controls were mounted on a pod on the steering column.
After 91, it was replaced by the Diamante.
My Sigma is currently sitting, waiting for a new alternator.
Motor and interior (click for huge pics)