General:
* Functioning effectively
* Operating smoothly
* Performing efficiently
* Running smoothly
* Doing a good job
* Succeeding
* Thriving
More specific:
* Producing excellent results (if focusing on output)
* Collaborating seamlessly (if focusing on teamwork)
* Being reliable (if focusing on dependability)
* Being productive (if focusing on output)
* Being efficient (if focusing on minimizing waste)
* Being effective (if focusing on achieving goals)
Informal:
* Ticking along nicely
* Working like a charm
* Running like clockwork
The best synonym will depend on the specific context and the emphasis you want to convey.