I have Floramax in my main tank right now. Plants seem to like it just fine, and it's got enough texture that roots can grab on fairly easily. I pulled a stem or two of my rotala rotundifolia a couple days after planting to see how it was doing, and a couple pieces of rock were already hanging from the roots.
I believe it's nutritionally inert, so you will need some root tabs to feed your plants. Check in the For Sale forum here; I think one or two people have Osmocote gel caps listed right now. My plants love them. Right now I have a lot of stuff growing...green cabomba, rotala rotundifolia, R. indica, blyxa japonica, alternanthera reineckii, moneywort, dwarf sag, and a couple banana plants just having a huge time.
Currently I have it around 1 to 2 inches deep, but I picked up another bag at petsmart last night because I have some root-feeding plants (crypts, dwarf sag) that will be happier with more room to spread said roots. Also because I'm going to change my scape up a little and there just isn't enough in there now to work with for what I want to do, but that's another thread entirely.
Edit: I haven't used it with soil, but I would think that a layer about an inch thick over the soil would work.