Good letters don't depend on the medium, they depend on the writer and how much time they care to spend on it.
Correspondence is such a 19th century notion (really, the people from about 1840s - 1920s wrote the most amazing letters back and forth), post WWII the widespread electronic media really put a dent in people writing letter, so the 60s and 70s were very low periods. I think in a lot of ways the net and email has brought a lot of that back, and people have long conversations in a correspondence sense once again.