My first Garmin soft-strap worked for about a year and then started reading really erratic. I tried the suggestions for cleaning and it worked sometimes. I also had the problem with the fact that the strap was too big and wouldn't stay in place.

I tried DC rainmaker's suggestion with the Polar strap as I happened to have one from a Polar watch, it didn't work and I had the same erratic readings.

A couple of weeks ago I had to buy a new Garmin watch as mine was stolen and the new soft-strap has a slightly different design (it is actually more similiar to the Polar design) it also seems a little smaller and seems to fit me better. It is reading correctly now but it has only been 3 weeks so only time will tell :-)

The straps are expensive so I would expect them to last at least a few years on my old Garmin I used the same strap for a few years but perhaps maybe should't expect it to last forever.