Protesting property taxes

I have always protested my own property taxes with the Harris County Appraisal District, but the last couple of years I feel the results are not as stellar anymore.

While I showed them both recent sales and valuation of my direct neighbors, they hardly adjusted the value. I sincerely believe they over-value property and do so on purpose, at a high cost to me and many other families.

I am an European immigrant (10 years in Texas now) and never experienced a government this way. Dishonest, hostile? But my feelings are very not relevant for my question:

Does anyone have positive experiences hiring one of the many companies that offer help?