For some 40,000 Cubans, the arduous escape from a repressive and impoverished communist regime has ended. (By the time the last boat arrives from Cuba, the total could be closer to 80,000.) Most crossed the Florida Straits in small boats, many after days or weeks of anxious waiting in Cuba. Now they face what for most will be no less a challenge: carving out new lives in the US. They will not be without help. Many were met by relatives or friends when they landed in Key West; others have friends, relatives, or other sponsors in cities across the US. Since the takeover of the Cuban government by Fidal Castro in 1956, many thousands of expatriate Cubans have successfully established themselves in this country, with an especially vital community in Miami. They are the third-largest hispanic group in the US, the Mexican-Americans being the largest and Puerto Ricans, second. The Cubans, many of them well-educated and either experienced in business or the professions, have for the most part melded smoothly into the US society and economy -- in both white-collar and blue-collar jobs. Although the hard-pressed Castro government has tried to use the recent exodus to rid itself of people whom it considered undesirable, most of the recent wave of refuges apparently will be accepted by US immigration authorities and allowed to establish residence here.