I've been using the latest Garmin Forerunner 935 for a couple months and so far it's been reliable. We'll see how long it lasts. But realistically I think we're going to have to consider wearable devices as disposable after a few years.
Like with other tech, they tend to keep on working just fine, but you want to replace them because the new model has better features.
I upgraded from the 910 to the 920 to get the Bluetooth upload. That said, I'm not particularly tempted by the 935 over the 920, and I know people still using the 310XT which was released in 2009...
