I don't have long experience with SM, only about a month, but I do like it. I like the way it looks, the ease of planting in it, and the fact that corys can chew on it safely. It does grow plants and roots, but I have no idea whether it does it as well as, better than, or worse than any other substrate. And, I don't see it affecting the water parameters.
I love it. Its not as dark as I would have peferred but deffinitely very good. If you're looking for black, keep looking but for dark its great!. I'll post some pics when I find them but Matt has some readily available if he stops by.
The Soilmaster seems to be a very good substrate but it is a little "light in weight". That aspect doesn't bother me as much as it does some, especially when you can do a 75g tank's substrate for less than $20 It is lighter than Flourite, but I prefer the dark grey color over the multi color of Flourite.
It should also have a higher CEC than Flourite since this product is very similar to Turface. Here's a link for some substrate comparisons. Note the CEC and iron content differences of Flourite and Turface, Profile (The parent company of Turface) and Turface Black (similar to Soilmaster): http://home.infinet.net/teban/jamie.htm
Plants do just as well as those in my Eco substrate but they seem to have larger root systems in the Soilmaster. Maybe this is due to the lighter weight of the Soilmaster. I don't notice any changes in water parameters over my Tap Water either.