Has the Boeing 737 MAX MCAS system successfully prevented a stall or approach to stall condition outside testing?
We don't know - until we hear about such a case!

A positive answer would require 


a trace of MCAS action
sufficient evidence that the pilot could not have handled the (near) stall without MCAS.


This information could come from 


a pilot who


notices the trim interference, and 
perceives the situation as dangerous, and 
reports the event



or 


a flight data recorder that is analyzed, and


corrective MCAS action is found, and
it is evident from the data the pilot(s) did not / could not control the situation.



[ Am I missing any other way how MCAS could leave a trace? ]

A well-trained vigilant pilot can handle (near) stall situations in most cases.

MCAS is designed not only to get out of a stall, but to prevent the airplane from stalling in the first place. That means that
 the plane should never get into a critical stall situation, if MCAS does its preventive work.

All that just to say, it would be possible to answer "yes" as soon as there is knowledge of one averted incident, but it is impossible to conclude "no" from the absence of information.

I am very well aware that I wrote this theoretical answer while sitting in an arm chair, and I will happily remove it as soon as anybody with hands-on knowledge can provide more satisfying information.