It also speaks to prospective employers about a pattern of behavior. A single screw-up when you were in your early 20's is one thing, a string of criminal activity across several years is another. It does increase his difficulty in finding employment.
I'd think it would also depend on the crime and the role you are looking to fill. A felony fraud conviction would not help you find a job as a system administrator for a financial institution, for example. The help desk / admin side has access to passwords and information that is sensitive and any kind of background that suggests you might use that access inappropriately would be relevant to the employer's decision.
A database administrator would similarly be in a position to access all sorts of valuable information.
A string of convictions for assault and battery might hurt your chances in a team situation, or customer service situations, but would probably be easier to overcome than a criminal history that demonstrates a willingness to steal from the company. Then again, we had a couple of situations where unstable people brought the threat of violence into the workplace. With our lawsuit happy culture, you couldn't blame a company from shying away from someone with a proven history of violence. How quick do you think people would be to file a lawsuit if a company hired a person with a violent criminal history and there was an assault of some sort?
A developer position might be easier to get - depending on the type of job and the type of shop. Being the install guy for desktops or servers in a large shop would probably be a lot easier to get - they have tighter controls in place so trust isn't as critical as it is in a small shop where the guy installing the server is also the guy with all the passwords.
In any case, building some sort of history would be key. A good recommendation as a reliable employee for a couple of years would probably overcome a lot of objections - even if it wasn't in a relevant field. But it isn't easy out there for anyone, particularly with low experience, criminal record or not. My ex's father ran the food service at a university and he used his position to help a bunch of newly released prisoners get a fresh start. He said he only had a couple of guys disappoint him over a couple of decades. For him it was a form of ministry - but there are a lot of guys just like him out there who are willing to take a risk. Finding them is the trick, I suppose.