A new study by Iowa State University's Department of Kinesiology (referenced by the Phys.org website), paints a mixed picture of the accuracy the most popular products on the market, including the Fitbit Flex (£60), Nike+ FuelBand SE (£60), Jawbone UP 24 (£50) and Misfit Shine (£70).
Two research products not on sale to the public were also included for comparison, the BodyMedia Core and Actigraph GT3X.
The accuracy of the products when measuring sedentary, aerobic and resistance exercise using 56 test subjects was found to vary, with the BodyMedia Core and Actigraph GT3X+, perhaps not surprisingly given their sophistication, achieving the lowest overall error rates across the categories of 15.3 percent and 16.7 percent respectively.
The Fitbit Flex was next with an error rate of 16.8 percent, followed by the Nike+ FuelBand SE on 17.1 percent, the Jawbone UP 24 on 18.2 percent and the Misfit Shine on 30.4 percent.
According to Phys.org, which saw the study in detail, all of the brands measures calorie consumption reasonably accurately but each product measured specific types of activity to varying degrees of accuracy.
Whether any of this matters is open to some debate. The Fitbit Flex, for example, advertises itself as a way for users to track, steps taken, distances travelled, calories burned, minutes active, hours slept, and quality of sleep, and so he fact that the University found it to have an error rate of 30.4 percent on aerobic activity could be moot - it is not claiming to measure users under exercise load.
The motivational dimension of these devices is to offer a general picture of a person's real activity levels over time to counter the subjective view people have had up to now.
"I think the key to a consumer is not so much if the activity monitor is accurate in terms of calories, but whether it's motivational for them and keeps them accountable for activity in a day," ISU's professor of kinesiology, Greg Welk told Phys.org.
Investors remain convinced that wearables are more than a fad. In June Fitbit's IPO share price was $20, which soared to 30.40 by day one, valuing the company at $4.1 billion. After another surge, by it had hit the $40 mark.
Apple bet that the wearable market would be boosted by the iWatch, ostensibly a timepiece with added features. Fitbit's success suggests that, so far at least, fitness is the real killer in terms of ubiquity. For now, simpler, cheaper and longer-lasting devices are winning the argument.