To actually answer metel3d’s question, no - neither octrees nor the use of geomipterrain fix falling through the terrain - that is a failure of the collision system where you fall through a crack between the edges of polygons. The usual solution is to use multiple collision detection calls, so if one fails another will stop you falling through. (i.e. fire down multiple spaced out rays for instance.) I would use the geomipterrain to speed up basic rendering though.
Treeform: There are many situations where using octrees for your visible environment will result in a useful speed up - its dependent on the environment in question. This is especially the case for large dense environments where forward visibility is limited - there it could be the difference between playable and not.