Hilarious, as you'd expect with Harley
Some stodgy fans might consider the humor of this movie sophomoric, while I consider the jokes "modern".
A great homage to the Arleen Sorkin version (original) of Harley, fans of the Animated Series should love this, but it should noted that, since those fans are older now, the content is a bit more "adult" (an obvious, but not really graphic, sexual encounter between Harley and another character--I won't spoil it!).
Some, I've noticed, really didn't like the ending-- I thought its simplicity was appropriate. This is Batman and Harley Quinn, not Batman and Mr. Freeze, after all.
The animation style is great, the writing is above competent and the casting of "Animated Series" regulars (Melissa Rauch voices Harley, Paget Brewster as Ivy) makes this one a "must-have" in the collection of Dark Knight Fans!