yes they do improve performance and stability and you dont have to buy any of them. I use a free one that you can get from the link below http://www.hoverdesk.net/freeware.htm
Its fast and simple, i've been using it for a few years now without any problems.
I have been using Reg Cure, but that needs registration. I have downloaded the regSeek1.55, I would like to see if still worth to pay for the Reg Cure.
Wise registry Cleaner V7.48 Very easy to use for novice users and pros alike Wise Registry Cleaner will automatically remove and fix all your Windows Registry problems. ;D