They are slow growers, but if your cichlids don't mess with them too much they will eventually grow very big. It will tend to grow in a huge clump, and also grow baby plants at the tips of older dying leaves. You can pull off pieces and spread them around the tank. Its actually good for the plant to thin them out when they get very dense.
Unlike Java moss- they float, so it can be a little more of a pita, especially if your fish mess with it. Java moss just sinks and spreads out on its own as pieces come apart. the fern will float to the top and swirl around in your filter output.
Make sure your rizome isn't buried in the gravel- jam it in between rocks (be careful not to smash it), or tie it down. You can plant the roots if they are long enough, just not the rhizome (the meaty part that the roots grow down from and the leaves grow up from)