I have been on an ancient historians bender recently. If we judge Sallust by the standards of his fellows he is clearly one of the weakest, but so far as I can see nothing bad managed to survive the dark ages to come down to us from ancient Greece or Rome. The book is worth reading as a result, but is hardly the place to start.
Really the book is two short monographs on two important, but not very important incidents in the history of the Roman Republic. One of these monographs is on the war with Jurgurtha which took place in Numidia and is a typical minor war of Roman expansion. The other is the shorter work on the Cataline conspiracy which is the superior of the two.
In both works Sallust is a better writer than a historian, but in the Cataline conspiracy he tells a better and more compelling tale. In both works Sallust sees the incidents as showing the decline of the Republic following its enrichment in the wake of the Second Punic War and the conquest of Greece and Macedonia. I do not think that Sallust is wrong in that, other than in the moral component that he inscribes to the actors, which like most ancient historians he saw in continual decline. I don't see that myself, but certainly Empire at first changed and then destroyed Republic government.
He is a decent writer. The Conspiracy of Cataline in particular is a good yarn, but its only for those who have my weird enjoyment of reading ancient historians.