I have used a bit of OF. I got a demo free bottle of overgrow. Then I got a pack of the 4 small (250ml I think) bottles. Transport, WATTS, Attack and Rev. I have been using them mixed as overdrive quite a bit. Every other day on some plants. I have also mixed epsom salts with Transport to make up for a Mag def.
I can't say that I've noticed a lot using the product so far. But I am pretty new to reading plants. I look at it as something I should be doing and using and understanding because it just makes sense and the science proves it works and a lot of experienced growers know it works, which is good enough for me to keep using it and trying it out in different ways. And watching what others are doing with it.