I agree with Maximinus Furtivus' comments about Cicero's bad press.
I previously used Cicero's and Pliny the Younger's letters in my high school Latin Prose classes. Pliny purposely tried to copy Cicero's style but failed. His letters sound too artificial because of his attempts to emulate Cicero.
I particularly enjoyed having my students read Cicero's letters from exile. He aches to return to Rome to be "near the action".
His letters to his slave/secretary Tiro were touching. Tiro had gotten ill when the group travelling with Cicero had to return to Rome without him. Reading this letter, you can sense the closeness between the two men and Cicero's genuine concern for Tiro's recovery.
One letter of Pliny the Younger tells a ghost story - about a philosopher who rents a haunted house in Athens. Students enjoyed this story - especially around Hallowe'en.