Initially, I had some problems with mine.
In the foot sensor, the battery leads were joined onto the circuit board at a tight right angle and after a couple of weeks broke off with movement. I replaced these with slightly more sturdy leads myself (yes, I know it invalidates the warranty ;-).
I also had problems with the batteries working loose and breaking contact in the foot unit, cutting the power. I got round that by packing them in with foam padding (cut out of a shoulder pad bought from a dressmakers shop for a few pence!) underneath and above to keep them held in place.
Once I was aware of what was going on it was easy to fix and I've since had loads of use out of the SDM - it's been very useful and I use it on all my training runs. I've also checked it against OS maps and on the track and it's very accurate.