We had this question answered during our recent Fall of the Republic AMA here and here. In the latter link /u/Celebreth adds some damn good points to my usual ramblings on Catiline.

Certainly Cicero inflated the Catilinarian Conspiracies to enormous levels--after all, in every speech the guy made for the rest of his career he mentioned having saved the state from Catiline. Our two important sources for the Catilinarian Conspiracies are Cicero and Sallust. Sallust is no fan of the Catilinarians, but he inserts Caesar's speech against Cicero's decision to execute the Catilinarians without trial and presents generally a more balanced view of Catiline. Whether the Catilinarians were quite as big a threat as Cicero made out they certainly were a very real and very present threat. Their stated intention, from everything that we know about them, really does seem to have been to take over the workings of the state, either by political maneuvering or by force. As /u/Celebreth mentions, Catiline had amassed an army of about four legions (the size of a consular army) in Eturia just a hop, skip, and a jump away. This was an army composed mainly of Sulla's old veterans, who were pissed as hell that the senate had taken away the generous land grants that Sulla had given them in Campania (worse yet, they were actually living there when the senate took away their land). There was only one place for that army to go. Related to that, it's important to note just how widespread support for Catiline was throughout different levels of Roman society. Although we can't know how strong it was in any given stratum (and the fact that out of all of Sulla's veterans Catiline only got enough to support him to form four legions suggests that we should doubt the support of any one group) Catiline had support included a large number of nobiles, non-nobile patricians, Sulla's veterans, the so-called urban poor, and apparently many of the Italians. That's pretty significant, that so many people from such different circumstances were willing to follow him.

Another thing is of course the aftermath. No matter how dangerous the actual conspiracy was, what happened after it was exposed was pretty serious. In particular the execution of the Catilinarians without trial--an act in violation of Rome's oldest and most fundamental law, that Roman citizens could not legally be killed without trial. Caesar spoke out against this and almost swayed the senate, until Cato jumped up and shouted "Naw, kill 'em all!" Cicero prided himself in being a staunch defender of the Sullan constitution, though he thought that in many places it was unjust (but necessary for stability and peace). He mentions this several times in In Catilinam alone, and it's a major theme of both his rhetorical and philosophical works. Yet here he is, violating one of the most fundamental parts of the constitution...in defense of the constitution. It's easy to see why immediately afterwards the weight of the constitution starts to unravel very quickly, when even its most important supporters are willing to break its letter to defend its spirit.