I have a major issue with books that are open-ended (they don't finish). I like it only marginally better when a movie does this, and I think it should be required in either case to tell people in advance "we're not going to wrap this up." You can make a book or a movie that provides for the possibility of a sequel while still wrapping up your plot threads. Or you can say "Fuck You" to your audience and leave them with a heap of unresolved issues that will only be answered if your movie is popular enough and makes enough money to warrant a sequel. That's "Alita," ending with the big middle finger to the audience. "Here's the real big-bad, and maybe Alita will get at him in the next movie ... or five movies after that."
Now that I've got that out, maybe I can write a review based on the merits (or lack thereof) of the content itself.
If you've seen the trailer, it's kind of hard to miss Alita's giant eyes. And they do occasionally pass into the uncanny valley, but for the most part the special effects are very good. The 3D (BluRay, not theatre) is good, although I did notice some artifacting around moving foreground objects - I was surprised to find that the movie was apparently shot in real 3D rather than being done in post. (Also very surprising that I could get a 3DBR: I thought the technology was officially dead for the home ...)
Alita (Rosa Salazar) is a cyborg, her head and upper torso recovered from the junk heap under the floating city of Zalem - the place that pretty much everyone in Alita's world aspires to live in. Alita is reassembled by Dyson Ido (Christoph Waltz), a cyborg repairman. She turns out to be sweet, impulsive, and irresistibly drawn to battle - it's what she loves best. This leads her to the spectacularly brutal sport of Motorball, where cyborgs tear each other apart (and incidentally try to score points with a ball). But bad politics in the city and her own history (which she can't initially remember) lead to her becoming a lightning rod for trouble both inside and outside the Motorball arena.
SPOILER ALERT: I'm going to bitch about the spectacularly poor logic of the ending. Stop reading if you don't want to know about that.
Alita has claimed "I do not stand by in the presence of evil," and so it's implied that she's on a quest for justice against Nova, the leader(? we don't even know for sure) of Zalem. And Nova directly threatened everyone she cared for if she pursued him. And yet, "several months later," at the end of the film, she's directly challenging him from the Motorball arena - presumably on the assumption that winning Motorball is the only way to Zalem. Wait, what? Who set that rule? The management of Zalem, meaning quite likely Nova. And how did the last returnee to Zalem go? In little meat pieces. Oh, and to get there, she has to have played multiple rounds of Motorball, damaging or killing dozens of other cyborgs who (while they may not be innocents) don't deserve to die or have to pay huge amounts of money to get their bodies fixed because of her thirst for revenge. That's not justice, and she claims to be in favour of justice. And if Nova has been letting you play Motorball for several months without harassing or killing those close to you, then this is his game. He wants you to do this. And even if Alita isn't smart enough to work that out, her "father" is. She's had several months to think about it (I worked it out in seconds). So not only is the movie setting up a sequel, it's saying "our heroine is a moron." Not a great start - or end - from my perspective.