As a companion volume to Cicero's Letters to Atticus, this allows a broader view of both Cicero himself and the society around him. Unlike the Atticus volume this does include Cicero's letters around the assassination conspiracy and show him blatantly encouraging Brutus to murder Caesar.
The Renaissance is probably responsible for the view that Cicero was the great defender of democracy, an idea which still has currency today. These letters show the other side, though, (as do texts such as Sallust's Catiline) giving us a Cicero who can be small-minded and petty, whinging and vacillating, pompous and self-aggrandising.
So regardless of whether you're a Cicero `fan' or not, this are both crucial historical documents as well as a fascinating glimpse into classical Rome.