What are the common issues and best practices when using ASP.NET session state?

Dim sessionObj As Object = CType(Session("SessionKey"), Object)
If sessionObj Is Nothing
    sessionObj = ReCreateObj()
    Session("SessionKey") = sessionObj
End If

object sessionObj = Session["SessionKey"] as object ;
if(sessionObj == null)
{
    sessionObj = ReCreateObj();
    Session["SessionKey"] = sessionObj;
}