A good quality radiator sealant additive does help seal a radiator that is leaking and can help get your vehicle going down the road until you can replace the radiator.
If your radiator is leaking and you need to use a radiator sealant that's okay for a short while but you should really replace the radiator with a new one eventually to prevent further problems and possibly risk having the radiator blow out which can sometimes happen.
I've used radiator sealant in my car before with good luck and great results but eventually I went ahead and replaced the radiator.
The radiator stop leak additive I used was Bar's Stop Radiator Stop Leak and it worked great and stopped my radiator from leaking.
I would highly recommend using it if you have a radiator leak.