Step counts are fairly accurate. From the Flex less so than their other devices, but still not too bad. Nike Fuelband is pretty bad. Pretty good figures at the link below.
Case MA, Burwick HA, Volpp KG, Patel MS. Accuracy of Smartphone Applications and Wearable Devices for Tracking Physical Activity Data. JAMA. 2015;313(6):625-626. doi:10.1001/jama.2014.17841. http://jama.jamanetwork.com/article.aspx?articleid=2108876
Case MA, Burwick HA, Volpp KG, Patel MS. Accuracy of Smartphone Applications and Wearable Devices for Tracking Physical Activity Data. JAMA. 2015;313(6):625-626. doi:10.1001/jama.2014.17841. http://jama.jamanetwork.com/article.aspx?articleid=2108876