We have none registered in our area as far as I know (and according to registries) so there are areas where there aren't any "known" offenders, but keep in mind, in order to get registered, these people have to have been "caught" offending many times.
With that said, there are no guarantees that any neighborhood is safe. You could have a freak next door right now, or the next time you have a repair guy stop by to fix something, invite one right into your home.
This isn't to say offender registries aren't important or helpful. They are very important and helpful! It is far better to know who is living near you and your children than not, and the registry can lessen the chances that someone in your neighborhood will be victimized because everyone will know he is there. Hopefully she/he'll think twice too. But once again, there are no guarantees. Registries are just a help to keep you aware and informed. And police will know where to look if anything does happen like a child goes missing or someone is attacked.
You may be able to find out what these guys did if you go to your local Bureau of Criminal Apprehension. I'm not sure if that's what all states call it, but generally, all criminal records are public information. If you have these guys full name, date of birth, you should be able to go to the state data base, or at the very least your county courthouse and run a background check on these guys and get the full run-down.
Another place to look is metro and neighborhood news paper archives. While their crimes may not have occured in your state/area, you might still find something from when it was announced to the neighborhood the offender was moving in. Usually, residents are informed when an offender will be moving in and sometimes there will be neighborhood meetings or hearings. That information should be on record somewhere also. City Hall might be a good place to check. At the very least, the local neighborhood association will definitely know everything. If you can find the contact for that, you'll probably get all you need to know from them. The community news paper is probably a good way to get the neighborhood association's phone numbers.
Lastly, is it a good idea to buy a home in a neighborhood where there is a listed offender? JMO, I'd say no way! Not only are you being warned someone known to be unsafe and untrustworthy will be living next to you and you have the spend the rest of your days on hyper-vigilence. Will you ever feel comfortable sitting in your yard, or letting your child ride their bike or walk to a friends house? Who needs that stress? Furthermore, offenders are often clustered in areas where the housing is affordable, but then you have to ask why is it more affordable? Why are they clustered there? Is it because of a high crime rate, because of a lack of neighborhood involvement, etc? I think it can you alot about where you're moving.
If you look at state registries, you'll see the patterns. They're not supposed to do that, but that seems to be what happens. Besides safety, I'd be considering house value/devaluation if I ever wanted or needed to move, what this says about the neighborhood culture in general, etc.
I hate to generalize, but unfortunately, this sort of thing can definitely affect the culture of a neighborhood. Especially if the community isn't a strong one that sticks together. Do more research or look elsewhere I say.
To be honest, I think if you always exercise caution and stress safety to your kids, that's the best protection. As for buying a house there, if you could live elsewhere, it would probably be better...but for other obvious reasons.
Just some food for thought.