there's a post about sportlegs on mtbr.com in the xc racing and training forum. most say they work, especially for cramps. cramps have never been a problem for me and i them. i love 'em, they definitely take the edge off the burn during races.
I've tried it and felt good during the efforts. People can only give you their anecdotal accounts of whether or not this works. I haven't found any solid science to prove claims, but it's cheap and I believe it helps so this may be enough to have an ergogenic effect. The placebo effect can be powerful.