I just purchased a FitBit Alta and was disappointed that S Health supports a limited number of activity trackers and smart watches. (I suppose it is to continue selling their own brand of fitness products.) Before I switched to the FitBit app, I used S Health to log steps, calories, activity, water intake, weight.
Overall, I find my biggest gripe to be lack of compatibility with devices of our choosing. If it did support FitBit, I'd definitely continue using it as the dashboard is clean and easy to access. I also like taking pictures of my meals when I log them, so that is an added bonus through S Health. The step tracker seems to be pretty accurate for me while walking and on the elliptical. When lifting weights, I really don't worry about tracking that activity and whatever steps are included is fine. The sleep tracker asks you every day if you were asleep for a specific duration and to assign it a rating.
The app itself is very rewarding to use and you'll get encouraging notifications as you meet goals and set personal bests. It's not perfect, but if you can live with the little nuances that come with almost every app, then you'll be just fine. The more you use it, the better analysis and trend data you will get.