A number of years ago we did a series of articles about paddle design in the Sea Kayaker Newsletter. These were done in the format of a series of questions, each answered by a number of paddle designers. They are still worth a read.
Part 1 - May 2007 - http://www.seakayakermag.com/2007/07e-newsletters/May/paddles.htm
Part 2 - June 2007 - http://www.seakayakermag.com/2007/07e-newsletters/June/paddlesPart2.htm
Sea Kayaker Magazine