My PowerTap will probably fit the bill for me, but only having one for a short time I can't really say I have "never gone back." For me the biggest thing that has stood the test of time is a GPS watch. I starting wearing one in 2011, and probably have done less than 5 runs without GPS tracking since the that day. I can say without a doubt that wearing a watch while running makes me faster, it's the whip I need to make me push myself. Without a watch I get lost in pace and distance and tend to lose focus and motivation.
Powertap / Cycleops / Saris
Powertap / Cycleops / Saris