I've set up two tanks using the organic choice potting mix.
First was unsifted, and after a couple/few months, I started noticing a couple chunks of wood/twigs/bark, etc. on the surface of the substrate. Didn't really bother me any, but it might bother some people.
Second I sifted, and it's only been up a couple months, so I can't really say much so far.
There was some initial bubbling at first, I think a combination of air trapped during set up (I can still see some pockets near the glass here and there), and some of the rapidly-decomposing components. I did a fishless cycle both times, so if it was leaching any ammonia, I didn't notice it during the cycle, or the cycle accommodated whatever rate of ammonia production might of been happening.