Let me state that I have never used it, but I have clients that have.
Here is the scoop. You can apply almost anything to a basement floor to stop Radon. The question is...how will it last over time. Meaning, if you painted the basement with latex paint, then did a Radon test, the Radon levels would be lower. However, over time the latex pain will not last and be an effective barrier. So, I do not know how well RadonSeal will be over time. Most people will test the Radon levels right after an application, but not 3 years later.
Do not forget to address all cracks, for cracks are considered the path of least resistance for Radon.
No matter how good a product is, proper surface preparation is the key.