No personal experience. Unfortunately, the assay procedure used for humic acid and fulvic acid are defined by solubility in acids or alkali. They don’t tell you anything about the chemical structure which is the critical issue because these kinds of products work by binding minerals (both anions and cations) and increasing their bioavalibility to the plant. They can be derived from many sources - peat, coal (in its various grades), etc. Unfortunately, the three-dimensional structure and the binding capacity is source dependent.

If your plants are not seeing a nutrient deficiency during the growing season, then increasing nutrient availability will have no effect. If they are seeing a nutrient deficiency, then you may or may not see an effect on growth – depending upon the effect of humic acid on that particular nutrient.

So results are field (soil), humic/fulvic acid source and crop specific. The only way to determine its impact on productivity is to test  a product from a specific source under your conditions.  

One way to test usefulness is to tell the salesman that if he/she supplies the material to turn a field into a full scale strip trial, you’ll apply it and then split the difference in crop productivity, valued at harvest, with them.