The first Pulse activity tracker was well received due to its ease of use and sleek design, and the Pulse O2 improves on it by adding a new blood oxygen sensor and wrist strap. While it may not seem like much, the new strap makes a difference -- Stuff thinks "it looks more high-flying executive than casual gym slob," and The Gadgeteer says it's "easy to put on and take off unlike Fitbit Force straps which are annoyingly difficult."
However, Withings failed to improve some key features on the O2, namely the sleep and fitness tracking. PC Mag notes "you have to manually enable and disable sleep mode when you go to bed and wake up, and Stuff had problems with its accuracy, finding it "pretty wonky" at measuring distance. PC Mag also says the "the heart rate monitor/oximeter just wasn't reliable."
However, these flaws are not specific to the Pulse O2. When you take into account the low cost of the device they should be expected -- especially when compared to some of the O2's higher end competition. But the Pulse O2 still does plenty right, and CNET says it's "still one of the best pure pedometers out there" even if "it's not a big leap forward over last year."