Hi John,
Yup, I've posted about my own experiences with the SDM on other threads. It is very accurate and repeatable but I did have some teething troubles getting it working when I first got it. That said, other folks have posted about their experiences and don't seem to have had any problems at all.
One thing to point out, though - the business end of the SDM is a footpod that you tie into the laces of one your shoes. That footpod is, according to the instructions, "splash resistant". I've used it successfully running through puddles and in heavy rain but I don't think it'd cope with being completely submerged in mud or water for long.
Dave