I wanted a smaller big chainring, but the design of my frame prevented me from lowering my front derailer at all.

I went from a 52t big ring to a 50t Biopace big ring, and carried over BioPaceII rings to the middle (38t) and granny (26t) positions.

I used this bike for practically all of my racing since late 1996, and am very pleased with the BioPace, which I feel allows an easier re-mount on difficult terrain as the cranks can get moving easier initially (i.e. the mid-stroke of the pedal moves a little further for a given road speed).

I use the rounder BioPace-II rings, which have only a 3.5% variation in gear ratio as opposed to the original BioPace with fully 8% variation.

The later BioPace-HP and BioPace-SG rings also were rounder at only 3.5%.

All BioPace big rings are only 3.5% variation to keep the front derailer in better control of the chain when shifting onto the big ring, and to allow a faster spin at the top end of the highest gear.

The rounder BioPace rings are very close in feel to round rings, and can be spun up to a high cadence. I like sprinting on these.

The bike in question is here, used for CX, XC, crits, hillclimb time-trials and quite a few centuries.
I've built 4 different sets of wheels/tires for it. The weird front derailer mounting is obvious.
More at: http://www.flickr.com/photos/18079002@N04/page9/