Avid Harry Potter Fan!
I watched Fantastic Beasts thrice, let me tell you this. Go watch it. Enjoy the movie for what it offers, not what you want it to offer or not for the comments of the supposed "Harry Potter fans" or the 'critics' who has issues with casting.
90's kids, Watch it to remind yourself about a big part of your childhood.
Others, you won't enjoy the movie as much unless you have read the Harry Potter Series (at least Death Hallows - preferably more than once ;P).
Are there plot holes? Yes. Because fans came up with theories but J.K Rowling did not want to stick to them. Deal with it. As for Nagini, read Rowling's twitter.
ABSOLUTELY LOVED EVERY SINGLE MOMENT OF THE MOVIE!