I think it depends on the criminal. Were they committing the crime because they were financing an addiction, do they enjoy hurting people, was it a lifestyle for them (gang bangers)?
Sometimes taking a drug addict out of their environment helps to get them clean for a while, but I've seen addicts have a higher recidivism rate because the addiction is never actually addressed by incarceration. People with a sexual deviancy (rapists, pedophilia) I don't believe can be rehabilitated.
It is interesting to note that most people charged with murder do not go back to prison. In most women's cases the crime was usually a self-defense case. Sometimes it was a complete lack of judgement. If you have even one beer at a bar, then get into an accident that kills another person (even if they were the one that ran the red light, etc) you are at fault and in many cases the state will press charges on you for second degree murder or manslaughter. Do I think those kinds of "criminals" will get out only to kill again? No.
I did five years in the pen for accessory to murder. Long story short, I refused to testify against my boyfriend who beat up that guy who was raping me. Unfortunately, he died from his injuries. My choice was A) To testify against him and him receive a life sentence or B)Not testify, he gets a lesser sentence, and ruin a good chunk of my life. While I am a "criminal", I was an upstanding citizen before the event, and I see no reason why I wouldn't be now.
So it depends on the definition of criminal. Do you mean hard core gang bangers or do you mean ex-offenders who made a bad judgement call?