I thought Leto was good, interesting to see he kind of talked like Heath Ledger, but I didn't like how he appeared so little in the movie.
I also wasn't really satisfied with how the movie portrayed Harley and Joker's relationship. In the cartoon series, Harley genuinely loved Joker, but Mr. J, not so much, he constantly abused her, hit her and mainly used her for his own gains, he just sometimes pretended to love her so that she can easily follow his orders.
The reason why that relationship worked was because of how toxic it is. Very appropriate for 2 very unstable people, with Joker being much more mentally ill and cruel.
Still, from the brief time I saw the Joker in the movie, he was alright, but I'll need to see more to have a true final verdict.