One of the features I was most looking forward to on the Apple Watch was the fitness tracking capabilities. I had considered buying other fitness wearables, but decided not to because I didn't want to feel like I was wasting my money. I knew that there was almost no chance that I would wear a Fitbit, UP by Jawbone, or something similar after I owned an Apple Watch. I was, however, prepared to purchase a fitness wearable if I had decided not to buy an Apple Watch (but seriously, what were the chances of that?) I wanted fitness tracking because I thought it would aid my weight loss and fitness efforts.
I use one of the Apple Sport Bands for my workouts (it's actually the only band I have.) I don't think I would like one of the metal or leather bands during a workout, but having never tried them I could be wrong. That said, Jennifer has the Milanese Loop and has worn it while doing yoga a few times and she really likes it.
Both the Apple Watch and iPhone include an Activity App (I could be wrong but I don't think the iPhone has the app until paired with an Apple Watch). This is where you can monitor your progress towards various goals. The three goals are active calories burned (which you can edit), total daily exercise minutes (30), and standing for at least one minute each hour, in 12 different hours The Watch can prompt you when it's time to stand, but if you've already stood during that given hour it will give you credit for it and you won't get the prompt (this prompt can be turned off). On Mondays it will give you a weekly summary of the previous week and ask if you want to change your active calorie goal. It will suggest a goal but you can pick your own if you'd prefer. You can also change this at anytime during the week. Your progress towards these goals can be checked in a couple different ways, the most common of which is just looking at it on your watch. It's represented by a graph with three rings. The outer red ring is your active calories, the middle green is your exercise minutes, and the inner blue is your standing. It will continue to track and record your progress even if you go past your daily goal. All of your activity data can also be viewed on the iPhone app. As you you complete goals over time, you can earn various achievements, such as one for completing seven workouts in a single week, or one for completing all three goals every day in a single week. I have already had a few days where I went on an extra walk for nothing else then I wanted to fill up those rings. In this way, the Apple Watch has effectively turned fitness into a game.
While I used to run on a regular basis (I did the Couch to 5K program, a couple times) I hadn't run in a long time. That said I have gone on some run/walks lately to try out the Workout app.
You can select from a number of preset workouts, such as Outdoor Walk, Indoor Run, etc. You can then set a goal for your workout: calories burned, total time, total distance, or open. After selecting your workout and goal you start the workout and let it run for the duration. I haven't seen any official documentation on how often your heart rate is recorded during a workout, but by looking back through my own data it seems to be roughly every five seconds. At the end of your workout (force touch then select "end" to finish) you are given an overview of your workout. This overview includes all the things that you could have selected as your goal, plus your average pace in minutes per mile and what your average heart rate was. According to what I have read, Chest Straps are the most accurate heart rate monitors available for consumers, but the watch has proven to be quite accurate. As I am not doing any kind of serious training, I don't need, or even want, a top of the line heart rate monitor. So, this is good enough for me. I like to see the number but don't stress about making sure I'm in target heart rate zones.
If you're curious, you can use the Heart Rate glance on the watch to see what your heart rate is at any given time throughout the day. This data can also be reviewed in the health app on the iPhone. From what I can find, unless you're using the workout app, it records your heart rate roughly every 10 minutes. Some people turn this off to preserve battery life but for the few weeks I've had the watch I haven't had any battery life problems.
If you want GPS tracking for your outdoor workouts you have to bring along your iPhone, as the Apple Watch does not include a GPS chip. However, after calibrating your watch, you can leave your phone behind and at least get both step counts and distance traveled. If you have bluetooth headphones, you can put music directly on the Watch and play it while you're on your run, even if you left your phone at home. At this time, streaming services such as Pandora and Spotify won't work without the paired iPhone being with you as well.
A downside to the workout app is that it's really only useful for cardio based workouts. You can still use it with other workouts by selecting the Other category. This will base your calorie burn off of a brisk walk. So no, if you're doing primarily a strength training workout, you won't get as much out of it for that. But you will still get the heart rate tracking, and workout minutes towards your daily goals. If you happen to use the food diary app, My Fitness Pal, the various workouts you do with the watch app can sync into your MFP account, recording the exercise calories.
Many people like sleep tracking capabilities that other fitness wearables offer. Sleep tracking is not available on the Apple Watch (it needs charged daily.) For some people, that might be a deal breaker, but for me I wouldn't want to sleep with something on my wrist anyway.
I can't recommend you purchase an Apple Watch solely for fitness tracking capabilities. However, if you want other features such as messaging, and running apps, while also using a fitness tracker, then it is great. For me, having the Apple Watch negates any desire or need for another type of fitness tracker. I'll probably continue my walk/runs if for nothing else than they are a relatively easy way to fill up those rings.