In a word, very!
Cuba has one of the lowest crime rates in the world, being a socialist country while there are shortages of luxury items there is very little real desperation, there is no homelessness or real destitution, food, electricity, gas, and accommodation are all covered by the Government, drug abuse is not commonplace either.
Crime does happen in all countries of course and whilst violent crime is very rare in Cuba, opportunistic crime can happen more frequently especially in bigger cities, be on the watch for pickpockets, always keep purses and bags with you, use the safe-deposit boxes in your rooms, only use official currency exchange desks, only buy excursions from our local reps or other official guides locally, only buy products like cigars and water from official stores, always be aware of your surroundings and most importantly apply common-sense at all times and you’ll have a wonderful time in Cuba.
At Love Cuba we strongly recommend that all travellers to Cuba take fully comprehensive travel insurance.