Sheldon Brown has a comprhensive article about these. Here are a few things he talks about. See articlr for full text.
Biopace Chainwheels
What Are They Good For?
Biopace chainwheels are particularly suitable for
touring cyclists and
time trialists, or any application that involves a steady, fairly constant
cadence. They allow healthy, efficient pedaling at slower cadences than is possible with round chainwheels. They are especially suitable for
triathletes and
mountain bikers. The triathlete benefits because the motion is a little bit closer to that of running, making the transition easier.
The mountain biker particularly benefits, because the Biopace design somewhat smooths out the delivery of power to the rear wheel. In climbing on loose surfaces, the limiting factor is often
traction.
[h=3]The
Cadence Issue[/h]The marketers of Biopace made a crucial error of judgment: too much information. In particular, they mentioned that the Biopace design was optimized for
cadences of about 90 rpm and slower. Many readers interpreted this as an indication that Biopace chainrings would somehow interfere with pedaling faster than that. This perception caused a bit of a self-fulfilling prophecy, and the Conventional Wisdom arose that Biopace was bad for high-performance cyclists for this reason. I [Sheldon] used to believe this myself, but I rode them anyway because I appreciated their other virtues.
[h=3]Biopace and
Fixed Gear[/h]People are often astonished to learn that I ride Biopace chainrings on fixed-gear bikes.
[h=3]Biopace Combinations[/h]The usual stock combinations with Biopace chainrings were 48-38-28 or 46-36-26 for a wide-step triple; 52-42 for a racing double. A
half-step-plus-grannies combination is possible with 50T-46T or 48T-44T for the two outer chainwheels and a 28T or 26T inner chainwheel. This works well with an 11-13-16-19-23-28-34 cassette -- a custom combination, to be sure
[h=3]Orientation[/h]For installing Biopace chainrings, the orientation should generally be preserved. Each Biopace chainring has an "indexing mark." This is a small tab pointing inward toward the middle of the chainring.
Mixing Biopace and Round Chainwheels
It is possible to mix Biopace and round chainwheels on the same crankset, but I [Sheldon] would not recommend it.
[h=3]Conclusion[/h]Cheers