'All letters are equal, but some letters are more equal than other letters'

This (sorry, George!) is in my opinion the truest that can be said of Cicero's letters to his very close friend Titus Pomponius Atticus.

Why is that?

As it is, all of Cicero's letters stand apart from all the other correspondence that has come to us from Antiquity. For example, the letters of Plato are generally considered to be fictions from a later time (though about the 7th letter opinions tend to a more favourable judgement). When one comes to the letters of Seneca, there's something very important that one must bear in mind: they were all written with publication as a final goal. So Seneca speaks to his friend Lucilius on all kinds of ethic behaviour and the art of living well according to Stoic philosophy, but he always had the general reader in mind as well. The same goes for Plinius (a.k.a. Pliny the Younger), who was possibly even more vain than Seneca; though an exception must be made for his letters to the emperor Traianus. But in these he is so grovelling that one almost gets nauseous when reading them.

But Cicero's letters to his friends (ad familiares), to his brother (ad Quintum fratrem) and to Atticus (ad Atticum) were what a letter is generally supposed to be: a way to express one's personal thoughts to a person one trusts. Of course, there are levels of confidence. Cicero is much more cautious when writing to Iulius Caesar, a political adversary, then in his letters to his wife. But they all have an important feature in common: no letter was ever intended for the eyes of another person than the addressee. And the pinnacle of confidence and open mindedness is the correspondence with Atticus, his closest friend and supporter. Here Cicero opens himself up as completely as one could wish. His many virtues AND less attractive personal aspects come to light. One could say that, dus to his very personal correspondence, Cicero is one of the few personae from Antiquity we know intimately as a person, a human being with all his fears and hopes, all his endearing weaknesses and ugly weaknesses.
I've always felt sorry for the bad press he has got. Just read the novels by Colleen McCullough and Steven Saylor, for instance. Little applause for Cicero there, alas.
If it wasn't for his surviving letters, his image might have been a lot better.
As for me: to know one is to love one. People have to come up with very strong arguments indeed to make me appreciate him less.

So read his letters. And get to know an important classical figure intimately.