Let's get the obvious out of the way: This is an adaptation of one of the most influential Batman stories with Kevin Conroy voicing Batman and Mark Hamill voicing The Joker. That alone is worth the price of admission.
The adaptation itself is done extremely well, but The Killing Joke isn't a very long graphic novel, so they decide to add something of a prelude to it. Most of that prelude works fairly well with Batgirl, Barbara Gordan, pursuing a criminal who is becoming obsessed with her. Batman identifies this and forbids her from getting involved. The whole set up and story of her and the criminal is an early parallel to the eventual "relationship" between Batman and Joker, one of obsession.
Where the prelude gets extremely weird is a one night stand between Batman and Batgirl. It's awkward, out of place, and just creepy considering Batgirl is Commissioner Gordan's daughter and has historically been in a sometimes relationship with the original Robin with Batman acting as a type of surrogate father to both of them.
I suppose since it ties into the Batman animated universe's continuity, one might say this is the event that drove a wedge between Batman and Nightwing. In Batman Beyond, I believe it was suggested a relationship with Batgirl was the reason Nightwing never returned to become Batman after Bruce retired, so die hards might find it somewhat workable to weave in, but for the average viewer, it feels odd and in the context of this film, very out of place.
But if you can set that aside, the movie is overall still really good and a great adaptation of the graphic novel, including a musical number by the Joker.