unless you have a knowledgeable source they are often fakes.
This is an understatement. Over 90% of the Cubans sold in the US are fakes.
Regarding the quality of Cuban cigars, there was a few year period between 1998-2001 where the quality was very poor. Their production goals were wayyy too high.
Fortunately, they have become much more reasonable, and are focusing more on quality rather than quanitity. The cigars that have been produced over the last 2 years have been fantastic and have great aging potential.
The problem with Cubans is that the tobacco isn't aged before they are boxed and sold. As a result, cuban cigars go through a sick period usually from about 6 months through 2-3 years. During this time, they have a noticable ammonia flavor. This is a part of the natural aging process, escpecially with stronger cigars. Some cubans take 5+ years to mature. During that time, the flavor profile for a cuban cigar can change dramatically. A Montecristo with 1 year of age on it will be much different than the same cigar with 5 years of age.
If you want to read more about this, the most extensive book on the subject is
The Illustrated Encyclopedia of Post-Revolution Havana Cigars
It is extensive and has lots of pictures, but it is expensive.
Cup