Whether it's worth it or not is really specific to you and your PC. When it removed some rubbish that it found in the registry, did you find that useful? Was it worth paying a few quid for what it did? How much would it be worth to you for it to clear out the rest of the rubbish it found? How much would it be worth for the peace of mind that you can run it again at any time in the future? Add up all those values and, if they exceed the cost of buying Registry Mechanic, then it's worth it
Alternatively, look at it the other way: what's the cost of NOT having Registry Mechanic? How much would it cost you (in labour and time) to wipe your PC and re-install Windows and all your applications and data in order to clean out your PC and start again with a fresh registry? Compare that to the cost of buying Registry Mechanic, consider how much Registry Mechanic will help your PC rather than wiping everything and doing a more thorough job and then you can gauge whether it's worth buying.
With regards to how good it is, I don't know I'm afraid as I've never used it. However, I do know that some on here have used it and found it to be useful.
