Welcome to FT !!
That seems a simple straight forward question but it is not; it is loaded and if you search you will see that the same question comes up periodically in all the other airlines.
It is hard to value miles since it depends on how you use them but I think most of us would agree that less than 1 cent a mile is a waste of your miles. In general I think miles are valued at 1.4 or more cents and if you use them for First Class travel on LH then they are worth more like 10 to 12 cents per mile (which for many of us is unrealistic since we would never pay the $ 15000 for a ticket but we are willing to dish out 135,000 miles)
If you are interested in continuing accumulating miles with USAirways then don't use them now. Hold off and use them when fares are higher or for off-peak international tickets.