First of all, you have to remember that it's dirt, so it's never really going to look "good"

Second of all, you might be discontent with the busy-ness (almost typed 'business') of the tile. Look at the tile in terms of noise: is it noisy enough? Too noisy? Look at size of the clusters. Too big? Too small? How about the spacing of the clusters? Too close? Too distant? All I can tell you to do is to just play with it until you find something you like.
Overall I like it, both in terms of texture and palette.