For serious fitness fans, the TomTom Multi-Sport Cardio is a tempting option. Although the price runs high compared to other wearables, TomTom offers accuracy when it comes to the built-in heart rate monitor. The watch will alert you if you need to speed up or slow down, to ensure you are working out in your optimal HR zone - easy, fat burn, endure, speed or sprint. While zones are calculated purely on age and are not user-defined, if you're not an elite athlete, that's probably not an issue. In terms of aesthetics, this wearable can feel bulky. On the plus side, the extra-large display means you can see real-time running, cycling or swimming data at a glance.