We keep tabs on our customer's experiences and, admittedly, have had a few problems with cadence sensor batteries. Early on, some customers were inserting the batteries with reverse polarity. I can recall two or three (of 1,600 or so Edge 305 Bundles we've sold so far) that arrived DOA with a bad cadence sensor battery. The cadence sensors themselves seem very reliable, overall. We've never had a return.
Garmin support is very good, IMHO. I've seen them replace units smashed in SUV hatchbacks and offer deep deep discounts for some that were simply lost. I've had customers with problems who have had new units overnighted to them from Garmin. Missing a part (a rare occurance)? You'll have one quickly and painlessly. In this age of sell it and forget it, Garmin seems intent on satisfying their customers and providing a good experience.
Have a great day!