This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Biopace chainrings have fallen into disfavor in recent years. They
are hard to "pedal in circles". The early Biopace chainrings were
designed for cadences of around 50-70 rpm, while most recommend a
cadence of 80-100 rpm. Newer Biopace chainrings are less elliptical,
but the general consensus is to (if you are buying a new bike) get the
dealer to change the chainrings to round ones.
Sheldon Brown has some information on Biopage chainrings at