Thanks.

From the Blender manual

Split/Keep Vertex Order When importing an OBJ it’s useful to split up the objects into Blender objects, named according to the OBJ-file. However, this splitting looses the vertex order which is needed when using OBJ-files as morph targets. It also looses any vertices that are not connected to a face so this must be disabled if you want to keep the vertex order.

As a rule of thumb, except in some rare cases, I'll always choose "Keep Vertex Order" when importing an OBJ from another package. As Lobo indicates according to the manual, if you do not enable Keep Vertex Order, then the geometry import will lose any vertex assignments that were given to it from the original export. Splitting the vertices will allow you to keep hard edges and some other things, but you will lose the vertex order. so morph targets or any other mesh properties which depend on vertex order will no longer work with the imported mesh.

Thanks guys.