True story this...
My eye was caught by these sticks on the Friday at the FLM Expo this year, so I sat down and the guy gave me a demo on my calf. Seemed harmless enough...until I had a go myself! I'm not sure whether I was a bit heavy-handed, but when I got up my calf had totally stiffened in the way it does when you pull a muscle. It worsened over the next half an hour and I half-limped around the rest of the Expo! I stretched it fairly thoroughly over the next 24 hours and mercilessly, it was more or less fine on the Sunday morning. I was bricking it at the time though!
Moral of story? DON'T TRY MASSAGE STICKS FOR FIRST TIME JUST BEFORE MARATHON!!!!