After seeing sky-high heart rates during exercise with the GW4's wrist sensor over the past few weeks, I decided to compare it to my reliable Polar H10 chest strap. Unfortunately, Samsung does not provide native support for external BT HRM's, so I used the SportyGo app with their BT module installed - works perfectly to connect the H10 to the GW4. That's where the good news ends. When not under exercise stress, the GW4 wrist HRM is somewhat close to the H10, averaging 5 bpm difference. But, when hiking and especially when running, the GW4 is far off and wildly inconsistent with itself. When running, I see 20 to 40 bpm inaccuracy - sometimes more - always on the plus side, giving really alarming heart rates. When cooling down, it'll eventually drop down to around the proper reading. There are so many similar reports from GW4 owners, it's not an issue isolated to my watch. My previous watch, a lower-end Garmin was far more accurate, making wearing the H10 sort of redundant. Even my old $50 Samsung Fit2 did exercise-based HRM'ing better than the GW4 - so Samsung knows how to do this! Bottom line: Samsung has to address this problem with their HRM and they need to provide the BT HRM connectivity that has become standard in the watch industry, for those of us who want to use external HRMs. If Samsung wants to play in the "health wear" market, they need provide a competitive product. ... View more