I've been training with Skratch products for about a year with very good results. I've had no stomach issues or any cramping. I would point out that you need to be sure and not use more than the recommended amount of powder per bottle. The acidity will rise significantly and will be hard on your mouth.
I raced Calvin's Challenge (12 hour race) a couple of weeks ago and was still happy drinking Skratch at the end of the ride.
Regards,
Daniel Pack
Knoxville, TN