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
http://www.sheldonbrown.com/biopace.html.

Some parts © 2009 Advameg, Inc.