The Collaborative International Dictionary of English v.0.48:

hit it off \hit it off\ v. i.
   To get along well together; to be compatible; to work well
   together; -- of people in interactive situations; as, the new
   chemist hit it off immediately with his supervisor. [Colloq.]
   [PJC]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "hit it off":
   accord, agree, agree with, associate with, be acquainted with,
   be friends, be inseparable, chime in with, correspond, cotton to,
   empathize, fall in with, fraternize with, get along,
   get along with, get on with, harmonize, harmonize with,
   hobnob with, identify with, interchange, know, reciprocate,
   respond to, sing in chorus, sympathize, understand one another