I've been using my Apple watch to track the calories I burn along with an app called MyPlate. I have also been tracking what I eat and like having a general idea of how I'm doing. I needed to lose about 10 pounds and it worked!
I recently purchased an Apple Watch and it's definitely kept me on track with exercising and staying active. I definitely noticed a huge difference of my activity level and it's been motivating me to go exercise more. I've taken it swimming, surfing, yoga, and climbing. I'm really happy with my apple watch. My best friend also has the fitbit tracker and it's been motivating her during climbing and running.
The cons of having the fitness trackers is you have to remember to wear it, charge it, and if you're not used to having something on your wrist, it can get irritating. Another con can be that they don't always track your activity level, depending on the fitness tracker. During yoga, I feel like I'm getting a good workout (suffering when I hold the pose for extended times or just doing core exercises) but the apple watch won't record it (unless I make it an activity exercise in the app or if my heart rate is high). Other than that not too many cons for me to complain about. I think it comes down to personal preference.
I bequethed my Apple watch to my wife 6 months ago. I liked it a lot, but was frustrated by battery life. I also had a couple of very long thru hikes coming up and decided to buy a Garmin 5X Plus from REI (that later went on sale, but they wouldn't match the lower price as more than 30 days had passed 😞 )
I'm in total love with the 5x Plus. Awesome watch -you can load GPX files on it for longer hikes and the battery life is incredible. The stats, including oxygen levels is fascinating. It also integrates with Strava which I use a lot.
My only downside is the watch is big (and also pricey) but I have zero regrets buying it- one of my best purchases in a long time.