Canklecat's post above is the best I've ever read on the topic of BioPace chainrings. Should be a sticky.
It's possible that the currently available oval rings---Rotor, etc.---work for at least some riders who maintain a high cadence; but as RChung notes in his post, there doesn't seem to be much in the way of convincing evidence one way or the other concerning that design. Oval rings oriented like the Rotors have been around since at least the 1930s but never established a permanent presence in the market, so there's that.
Common sense tells you that you want the chainring oriented such that the gearing is highest where the strongest muscles are recruited. Shimano's engineers, unlike the people at all the companies that have introduced oval rings designed as common sense would seem to dictate, actually researched the ergonomics of pedaling.
Knowing that legs evolved to run at maximum efficiency using a brief foot strike followed by a comparatively long rest during each stride, Shimano's engineers designed BioPace chainrings to facilitate similar efficiencies of pedaling: lower effective gearing (reduced load) and reduced time spent where the stronger muscles are recruited, higher effective gearing (increased rest) where essentially no work is being done. Brilliant, really.
At the time when BioPace was introduced, Shimano had not yet figured out how to explain such subtleties to their American market. I think it might have been just before they hired Wayne Stetina at Shimano USA. Those of us who were around in the bike scene in those days can remember what happened next: the bike magazines praised BioPace at first but then, after the magazine writers started hearing complaints from bike racers that BioPace felt weird for high-cadence riding, and especially for sprinting, they began dissing BioPace.
Shimano's insisting that any bike manufacturer who wanted Shimano groupsets had to accept BioPace cranks didn't help their relationship with the manufacturers. The bike sales reps then complained to bike dealers. Net effect: the market went from everyone wanting BioPace to no one wanting BioPace.
After the backlash, Shimano quickly released BioPace II, which is a watered-down version of the original design, but by then the damage had been done.
Too bad. BioPace was really designed for sport riding at moderate cadences, which describes the way many riders ride.