The Collaborative International Dictionary of English v.0.48:

insightful \insightful\ adj.
   1. having intellectual depth; having or showing an
      exceptional degree of insight[2]; -- of people or their
      comments.

   Syn: thoughtful.
        [WordNet 1.5]

   2. characterized by sympathetic understanding.

   Syn: discerning, understanding.
        [WordNet 1.5]

   3. having a keen intellect.

   Syn: prehensile.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

insightful
    adj 1: exhibiting insight or clear and deep perception; "an
           insightful parent"; "the chapter is insightful and
           suggestive of new perspectives"-R.C.Angell

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "insightful":
   all-knowing, apperceptive, appercipient, apprehending,
   apprehensive, astute, aware, cognizant, comprehending, conscious,
   discerning, discriminating, gnostic, intelligent, judicious,
   knowing, knowledgeable, mindful, omniscient, penetrating,
   perceptive, percipient, perspicacious, prehensile, sagacious, sage,
   sensible, shrewd, sophic, understanding, wise