Most of the reviews you have read may well be users who are more attracted to the interface and user experience than the actual workings of the product. There are several ways that products try to "clean" the registry, such as deleting references to nonexistent files. Realistically though, this can be risky and the performance gain may or may not be significant. It's very hard to tell what is or isn't being used in the registry, especially for an automated program. If you want my advice, don't bother, you may well do more harm than good.
Disclaimer: I'm a technical person, but this area is not any particular specialty for me.
I'm not sure exactly what you're looking for, but I use CCleaner. It allows you to backup the registry before "cleaning" it up. Might be worth looking at.
As many of the above suggested, use CCleaner - it's a nice and free utility that does a decent job. Personally, I'm not a big fan of CCleaner. I use Auslogics BoostSpeed 4 which does a better job cleaning your registry, and when I compared both of these programs, BootSpeed found a lot more unused registry entries.
You can download a trial version in Download.com or in any other major download site.
ccleaner man... tested safe and handy... i even use it for deleting virus and spyrwares.... if you know what to do
I'll also vouch for CCleaner which has a registry cleaner included. It's free, works well, and the developers are always working to improve it.
I'd recommend Registry Mechanic. With Registry Mechanic you can safely clean, repair and optimize the Windows registry in a more efficient way. The trial is time unlimited and removes problems found in 6 sections of the registry only. A registered version is required to remove all detected problems.
I use CCleaner all the time. It's free, fast, and does anything you would need to with the registry.