Definately worth it.  After using a pen/tablet for a while, a mouse seems pre-historic.  A pen not only gives you more control, it is much faster and easier to use than a mouse.   For pre-production, production and post-production, it will help you reach your fullest potential.  I don't think you'll find anyone who has used a pen/tablet that would want to go back to a mouse.  You can get a small tablet (mine is only 4x5) for around $200US.  It is an investment in your art.   -Drew