Its a bit like fine grit. I've used it before but couldn't really say what it was like as I'm really still learning. I'm using it again in a new setup so hopef'ully I'll see some good results. Just make sure you have a nice thick layer of gravel or sand to cap it off, and that you don't have any fish that like to dig, else it might get messy.
I've used it under sand and have it in my Rio under the sand at the foreground. It's very good that it didn't come to when uprooting and replanting plants but for plant growth IME it can't compare with Aquasoil.
I liked the Tetraplant, but seriously, if you are even slightly considering it, go with Aquasoil. It looks great, is easy to use and the plants grow like crazy! The only reasons for not using it, IMO, are the cost and if you have fish that like to chew the substrate over like dwarf cichlids. This is why I have a large area of sand in both my tanks using Aquasoil.
The tetraplant complete substrate... that you need to top with another substrate... not really complete then is it?!? I used to have this. It was in a low-tech set-up and was fine. The recommended depth I found was far too shallow. I found that when I tried to plant things it kicked up tons of mess, dust and brown stuff. Doing a strip down made my tank go dark brown cloudy.
I then used aquaclay. I will never use it again.
I am now using ADA AS. I have had the tank running almost two weeks @ 2.3 wpg with no CO2 and everything is growing lush with no algae! Plants love this stuff. Go for the AS!
I've tried it in a couple of tanks and, like Lisa, found it's a big problem if you get any into the water column.
I know someone with a 600L tank that's 50cm or so deep, he's got a 3-4cm cap of gravel over it and doesn't do much replanting. It's fine for him and the plants go well. With a shallower tank and/or less of a cap, it only needs one slip during a big water change or a bit of replanting and you've got ugly brown peat dust all over your plants. It's maddeningly hard to remove.
If you can avoid the dust problem, it's very economical. If you can't it's a pain.
I used this on my first tank and despite making numerous mistakes, plant growth was fast and lush. Recently though, I have uprooted a few plants as I didn't like their location and it was a mess. Fortunately, this major work was carried out during a water change so I was able to suck away most of the dust as it came up. My next tank is going to be using ADA AS for definite, yes it works it out more expensive, although once you take into account the extra gravel layer that has to go on top of the Tetra - then perhaps not that much more expensive.