It's a meh. Potential spoilers ahead.
And the main reasons are:
a) The bad CGI at the start of the movie broke it for me. And then, again at the middle, and at the end. CGI was baaaad. Really bad.
b) Ezra Miller's portrayal of the Flashes. They are mostly annoying.
On the positive, I liked Supergirl and the Batmen for the minutes they are in the movie.
And a definitive spoiler ahead: the whole "I will change the future by placing a tomato can in a shopping cart. No, I've learned my lesson. I will let everybody die, go back again, remove the can from the shopping cart... And place it on a top shelf". Why?
Finally, the post-credits scene. Again. Why?
It's like they were cutting their losses. Don't invest an extra dime on CGI. Just put it out as it is. We are losing money, anyway.