I remember back when The Hobbit: An Unexpected Journey was just finishing its pre-production stage and they were beginning on the actual production of the film. There was a lot of activity on the IMDB boards, rumors and debates ran wild speculating or wondering what we might get out of this films. I also remember in those days there was a prevalent rumor that a major character would be a female Elf named Itaril, allegedly played by Saoirse Ronan.
Many were outraged at this notion, especially as the rumors tended to build her up as being an overtly tough "teenager" and the sort of character that in the realm of fanfiction might be called a "Mary Sue". There were other rumors going around that she would form a romance with Legolass or one of the dwarves which somewhat understandably many were unsure of. I personally was among the few who tried to defend her, simply on the ground that we knew very little at the time and I wanted to see how she was handled before I made any judgments.
The rumors of Itaril were finally disproven when it was announced that Evangeline Lily had signed on to play a female Elf named Tauriel in The Desolation of Smaug and There and Back Again. There was still going to be a female Elf who wasn't in the original book, but certainly not the implausibly imperfect "teenager" of the rumors. Still, criticism of the character persisted. Even though it was already established that Peter Jackson was going to expand on the story and add in new characters to flesh out the world he is depicting, many complained that Tauriel just existed to add in a female presence to a book with an all-male cast.
Eventually, as we got closer to the release of The Hobbit: The Desolation of Smaug we got to see a bit more of Tauriel. There were complaints as she showed up prominently on all the posters (most likely this is because the previous film already got us acquainted with the protagonists, so they wanted to show off all the new characters we were going to meet here). The trailer also seemed to suggest a romance with Legolass, bringing back the fears of those who opposed Itaril.
Of course, it certainly wouldn't be the first time in Peter Jackson's films that a female character filled the role of a minor male character in the book. In The Fellowship of the Ring, Arwen's role was briefly merged with a minor (male) character in the book when she saves Frodo from the ring wraiths.
However, in the actual film I thought Tauriel's role was actually handled okay. It turned out the trailers had taken some dialogue out of context, as King Thranduil talks about (and strongly opposes) a romance between Legolass and Tauriel but there is no indication of them being anything more than comrades. Also, unlike Elrond or Galadriel, Thranduil's kind of a bigoted, close-minded, and extremely xenophobic jerk so I don't know how reliable a source of information he is.
The one part that I'll confess had me a bit uncertain at first was the relationship between Tauriel and Kili, but it started to win me over eventually. It's not entirely unbelievable that Tauriel could sympathize somewhat with the Dwarves (being the subject of Thranduil's prejudice herself, as it is mentioned she is not of noble upbringing) and her emotional connection to Kili was actually quite touching without going too far. After all, it's still Bilbo who ultimately has to save everyone.
Tauriel may sympathize with the Dwarves but that doesn't mean she's crazy enough to throw away everything by letting them go (it is implied she had to go through a lot just to get where she is now) and then facing Thranduil's wrath when they inevitably get recaptured (which the Dwarves would have been had the Elves pursuing them not have to deal with an army of orcs at the same time). She does rebel against Thranduil, but also does so knowing the danger he fails to see to the kingdom of Mirkwood.
Where they decide to go with Tauriel in There and Back Again I can't say for sure. It's likely her romance with Kili won't work out (for reasons that will be clear if you know the plot of the book), but perhaps she'll play a role in bringing together the five armies in the end. In fact, given the way she works with Legolass perhaps whatever happens will help him in the long run to fight alongside Gimli. Whatever they decide to do with Tauriel, I personally think she's a fine addition to the cast.