Sure--I think it's Michael Grant (although I'd have to check to make sure) who makes the claim that they comprised as much as half of all gladiators by the end of the Republic. We know from the Satyricon that free gladiators were certainly extremely common--they took a special oath to their lanista that gave up nearly all control of their own personal lives. Caesar's triumph in 46 featured Furius Leptinus, from a praetorian family (i.e. one whose members in the past had attained the praetorship) fought Quintus Calpenus, a former senator, to the death. Dio relates that Fulvius Sepinus, a sitting senator, petitioned to fight in full armor but was prevented by Caesar, though he allowed the equites to compete as they wished. That last bit is important--free gladiators were social outcasts, freed slaves (or retired slave gladiators), former soldiers, bandits and other outlaws, and generally the refuse of society. While occasionally you find upper-class men like the ones noted fighting in the arena (and to do so and prevail was, not surprisingly, something of an honor, at least by some), it was not particularly common and by many it was disgraceful and below the social class of senators or even equites (Tacitus, for example, damns Nero's games for featuring more senators and women fighting than ever before). As something of an indicator of the sort of free men who were more commonly employed as professional gladiators, Florus has this to say about Spartacus:

Nec abnuit ille de stipendiario Thrace miles, de milite desertos, inde latro, deinde in honorem virium gladiator

Nor did he refuse, who had been a paid soldier in Thrace ["miles de stipendiario" is rather vague--he could either be a soldier paid by his native army, or he could be a tributary soldier fighting for the Romans, as the phrase is sometimes used in Cicero to describe a certain type of allied contingent. Appian's "ποτὲ Ῥωμαίοις" is similarly vague], then a deserter, then a bandit, and finally a gladiator, on account of his strength

Florus implies that Spartacus was a free man when he became a gladiator, which is not what Plutarch and Appian say. Whether Florus' account is right or not, it's important to note that he's highly condemning of Spartacus in general, and Spartacus' early career is wonderful support for Florus of his general nastiness and brutishness. It should give us some indication of the stereotypical free career gladiator