I have and started using Seachem Flourish excel, since November '12, however I have used very sparingly, because 1) I am new to planted tanks, 2) I purchased only low to medium plants, 3) I have LED lighting. 4) inert-substrate sand w/small amount of clay.
So I only put like 1-2ml at a time, depends on how much water I remove every 5 days, (ITs a 20 gallon long) usually 8 gallons.
I can say my amazon swords, cryps, cabomba, roseflolia, anubias are all doing well.
I just planted dwarf hairgrass and water wisteria which is in shock mode I think..
I've also tried API Leaf Zone, for this tank and my 60 gal, and it's okay, but nothing really noticiable in plant growth.
PS In case theres question about my LED lighting, I have two "day white" strips, and called the manufacturer of Aqueon Modular LED lighting yesterday, and she advised these are actually 10,000 K. They' re also considering rewording or adding this information on their product line to inform consumers..
This may not have answered your question but I guess from my inexperience I am going to continue to use it and see how it goes..:)