I've mixed EcoComplete with the gravel when I converted a previously fish-only tank, and it worked out well for me. I have not used the Eco alone.
I recently took down a 29gal tank and moved the water/fish/plants/equipment into a 36 bow. I used black flourite
in the new tank, I love it.
I have not used "regular" flourite, but the owner of my LFS showed me what happens if you don't rinse it real well - it looks like your fish are swimming in chocolate milk every time you disturb the substrate.
With the black flourite, I didn't need to rinse very much at all. I put it in the tank, filled the tank about 3/4 full and stirred up the substrate real well. Drain the water, repeat. Drain again and repeat again. (The Python is the greatest invention ever!) Fill the tank slowly and add plants/fish. I had hardly any clouding at all.
Edit: I've only been doing the plant thing for a little over a year now. The EcoComplete/gravel tank is going strong. I did put a thin layer of laterite
under my black flourite, figured it couldn't hurt.
I've wondered as well how long a substrate will last.