I have not used any yet because I have so many products already, but I have a friend who uses the Growmore brand. Make sure the color of the extract is black or very dark brown because other extracts that have different colors or are even clear in color have gone through a different type of process that is inferior to the process the real dark colored extracts have gone through. I don't know the details, but the inferior process kills off too much of the good stuff.
My friend swears by it and he has some beautiful plants. I don't believe he used it past veg or early flower.