I have been a loyal Fitbit user since 2013 and I love their devices and ecosystem. They are easy to use and very durable.
Approximately 3 years ago I received a Fitbit Flex as a gift and I loved the device, but it had one major flaw, which is also the reason why I no longer have it. The mechanism by which the strap locks on is completely inadequate and while I was in Seattle I actually lost it because the strap came loose.
While in the US I got a Fitbit Charge HR, which has a strap similar to most wristwatches, which is a lot more secure than that of the Fitbit Flex. It also has various additional features such as a heart rate monitor and floors climbed counter.
There are a lot of things I like about Fitbit in general. They are not bound to any mobile ecosystem and works on Android, IOS and Windows Mobile. I also like the Fitbit community with badges that can be earned for certain milestones based on things like steps and floors climbed and also the challenges that can be entered against friends.
Both the Fitbits I have owned were rubberized, which make them feel rugged and durable. While mountain biking I am never scared that the device might get smashed or damaged, as would be the case with something that has a large screen (such as the Apple watch or even the new Fitbit Blaze).
It is important to note that except for the Fitbit Blaze, none of the other Fitbit devices are smart watches, they are fitness devices. And if that is what you are looking for, I would really recommend Fitbit.