"800 Score" is an online test prep company. To be honest, I don't know anyone who's used their services, so I don't have a basis for comparison. You can do a little comparison yourself, though -- compare their questions and explanations to those in OG11 and OG12 (which I recommend as the starting place for everyone).
In any case, I also don't recommend using only one test prep provider's materials, for exactly this reason; a single data point on a hypothetical graph of your performance won't give you much confidence in its validity. Every test prep company is approximating the GMAT experience, so to approximate your standing vs. the real GMAT, you should use several approximators.
