Well, I have my forerunner for 4 months now
I mostly run on a treadmill with power.
One thing I noticed is that the power drifts values a lot, like, I´m running at 10 km/h, and in some runs it show 290 watts, and others 360 watts, I´m wearing the same shoes, and dont believe I gained or lost weight enough to change the values that much.
Other example is when I start changing the pace, it never gets back to where it should be.