I have used series 2 for a handful of runs. I try not to check pace, distance, etc., so I can't speak to that. I used it for the full with no battery issues. The longer runs do start to skew from my garmin 405. 5k and 10k tracking were less than .1 difference. Full on the garmin - 26.62, apple nike+ - 26.89, so although there wasn't a big difference, who knows how accurate it was. No major diversions on the course. For the virtual half, more variation (% wise) cause I stopped at the international gateway bathroom 13.16 v 13.48. I'll continue to run with both for a while - apple watch simplicity and garmin accuracy/rugged.