After having a heavily planted tank going for about a year now, I can say that there IS a difference noticed after dosing this... with a caveat.
This product seemingly does nothing at first, but if you are consistent with the dosing for about a month, you WILL see noticeable faster and stronger root growth, not necessarily with stem and leaf however. I've started dosing this on and off for a few weeks to encourage carpeting, and acclimating newer groups of plants.
There ARE some plants that may not respond great to this however, I had some odd reactions with Val, much like how they don't get along totally well with excel and other glut additives.
I keep a bottle of this handy, it's low key at times, but well worth the price.
My title hedges a bit, because I haven't used Advance for more than a few weeks, but it seems to be doing good in my tanks. The first thing to understand is that Advance isn't a fertilizer, per se: Seachem describes it as "an all-natural, biologic growth enhancer for aquatic plants. Its advanced formula contains phytohormones, minerals, and nutrients that dramatically stimulate the growth of both roots and shoots in aquatic plants." Thus it's meant (as I understand it) to facilitate the plants' use of the nutrients in the water so they can grow better. I think it works. I use it in two tanks: a minimally planted 10g, and a heavily planted 20-long. In the 10g, I added some crypts and started dosing with Advance (plus Easy Green). Based on my earlier experience with crypts, the Advance slowed melting, while new leaves appeared sooner than I expected. In the 20-long, after a couple of weeks of daily dosing with Advance, I was seeing strong growth from plants that hadn't been showing it, including "aerial" roots from plants that had never produce any. Also, my red plants, which had been ravaged by algae, were suddenly algae-free. I attribute this to better nutrient uptake facilitated by Advance. To make a long story short, Flourish Advance has been added to my regular dosing regime for all tanks.
Regarding my purchase experience, this was the usual flawless transaction with Aquarium Co-Op. Cory's hired and trained a great staff, and I couldn't be happier in my dealings with them.