My initial guesses matched these other answers, but from the comments I've gone in a slightly different direction.
Fitness Tracker - I'm imagining the FitBit Charge, but most meet the criteria
The more complications I have, the more others want me,
The more features they have, the more people want to buy them
Yet my face is easily read.
Most have a simple display
I only had to tell you one thing and you’d be fine with me,
The specific model I mentioned tells the time, but others have a simple display telling you want mode they're in (Jawbone) or how many steps you've taken (FitBit Flex)
These days you look elsewhere for the same.
You can check the time on your phone, and get the fitness information from related smart phone apps.
Thanks to all the jewels I’ve been given,
All the additional sensors
I’m a little more punctilious when I’m at work.
The fitness readings get more and more accurate