We are thinking about taking one of the Renaissance cruises this fall. Obviously, I have the same questions about World Airways as you do. All I could get from their home page is that they are "presently utilizing a well-maintained fleet of 10 international range, widebody MD-11 and DC-10-30 aircraft". It is a 51-year old company. They began flying for Renaissance last August with one aircraft and will add the second one in April, and a third one in June, 2000.
The reservation agents at Renaissance told me to plan on staying in New York overnight before and after the cruise, because they can't guarantee that the charter will depart and arrive on time. It did not sound encouraging, plus it adds 2 extra days and $400-500 to our vacation ( good hotels in New York ain't cheap ).
When you get back, please let us know of your experience with World Airways. It might affect our decision whether to take the cruise or not.