Fi is indeed the new and improved Navi as she can offer more description of the enemies (and character bio) than just a simple "how to attack this creature", offering a choice for more advice and helping you search for hidden objects, more than Navi could offer.
That said, I don't think she's annoying as many people said, same goes with Navi. She can be redundant at times, but I just simply relax emotions and enjoy the flow instead of rushing forward towards the game. Fi's stoic personality and obvious hints to me are practically hilarious as she is comically serious when she speaks thanks to her lack of understanding of human emotions. The Bokoblin description is one that made lol'ed. I think the reason why people hate Fi or Navi is that they have little patience for talk and just want to play the game.