Also have already played the first part madly and I have to say Enemy Within is, as for me, the ideal complement for fans of the "predecessor". A few more new Aliens would not hurt but was completely XCOM: Enemy Within my expectations.
For XCOM fans a must anyway and beginner-friendly. (Who now as "new player" asks: What should I bring XCOM Enemy Within which I can only put your heart because it is the same game, only with very nice additions?.)