I"m with the OP - for the cost, it couldn't hurt... and any improvement is better than no improvement.
having said that, i'd like to know more. besides being nearly free, what are you looking for? mileage, top end, acceleration, offroad, what? what engine and trans?
i have a 2000 4.2LV6 with the POS mazda 5 SPD that gets 12 mpg in traffic with the 3.55 axle and 245/15 kevlar MT/R tires. I am seriously considering getting a lower gear set, but i have the same question - what should i switch to? I suspect my poor mileage is due to having to keep my foot in it during my daily commute because that engine has poor acceleration due to the 3.55's. But, as someone else here noted, if a 3.73 swap doesn't make any significant difference, then what's the point? what gearset would make a difference?