Saw the movie on saturday by myself, and my thoughts on it are... extremely mixed, honestly. I'm not sure how to properly structure this, so I'm just gonna list below the good/bad things about it that I can think of right off the top of my head (some of which may have minor spoilers, so watch out for that innit), starting with...
THE GOOD STUFF
The visuals are, as per usual with these films, utterly gorgeous. A lotta reviews for the film have used the "oh no it doesnt use legos all the time its lazy" thing as a frequent criticism, but for me I... don't get this? The realistic natural elements with minifig-made structures being integrated into 'em reminded me mostly of those wonderfully-photographed spreads you'd get in 90s lego catalogues with train layouts and castles and such, and boy do I have a love for those layouts. S' a good visual style.
I also really really like Ninjago City's general design, it's almost like Los Angeles in Blade Runner but brighter and more child-friendly and I really really like that. Good stuff.
The animation is also, as per usual, still utterly gorgeous. They definitely do "cheat" a lot more with popping off minifig arms and exaggerated movements than the other films, but every smear frame in the film is still made with 100% lego elements so far as animation goes it's pretty dang inventive.
Apart from one or two duff notes (I'll get back to this in a sec), the voice acting is mostly pretty spot-on too. Garmadon, Lloyd and Zane in particular are dang great, and the rest of the ninjas are pretty well-voiced too... when they do eventually speak, that is (I'll also get back to this in a sec).
The stupid pointless love-triangle from the show that serves no purpose other than to start shipping wars on tumblr is completely absent here, thank christ.
The character designs are several notches above Ninjago's usual decent standard and some of them (particularly Zane and Jay) are probably my favourite iterations of the characters design-wise ever.
Zane is best ninja bar none.
Right, now we've covered that let's move onto the more questionable bits of the film, with...
THE BAD STUFF
On imdb the film has a grand total of 13 (!) writers, and when it comes down to the plot... you can really, really tell this thing's had a silly amount of rewrites/changes through production and the final result feels more like remnants of several different story ideas blended together in the edit rather than having a clear idea of what they wanted to do at the very start. There's bits like the Good Morning Ninjago segment introducing the ninjas at the start where you can really tell that they had a ton o' finalised footage they'd removed from an earlier cut that they didn't know what to do with so they just haphazardly shoved it in there and it just suuuuuuucks. Talking of the first Good Morning Ninjago segment...
With storytelling in any medium, there's a general golden rule that should never, ever be broken with characters - show, don't tell. If you wanna get across to the audience that a character is shy or reckless or calm, then you should get that across through their speech and mannerisms so the audience can work out for themselves which character is which. What you absolutely shouldn't do in any piece of media whatsoever is yell directly at the audience which character is which and exactly what all their character traits are because it treats the audience like two-year-olds and is frankly an extremely lazy way of introducing characters. Naturally then, that's exactly how TLNM decides to introduce you into this world and, to put it bluntly, it's absolutely piss-poor and by far the worst part of the film for me. Tiny bits and bobs of exposition is one thing, but getting two news anchors to spend like 5 minutes yelling at you exactly who each character is and their relationship to Lloyd is just extremely lazy and awful in every single way and I just hate it so much and ugh. Showing all of the mechs and the inside of Garmadon's fortress in said news segment also completely removes any sense of surprise from their initial reveals later on in the film for the audience, which also completely sucks. The absolute worst part about it is that it's pretty clear from the unused footage in the ninja montages that they were gonna have a longer battle sequence at the start that would've introduced every character naturally through actions like every other good film, but instead they opted for this route because... why? Why did they do this? Why?!?
This definitely isn't a problem unique to TLNM (By the sounds of it the MLP Movie does it and I've got my fears about Avengers: Infinity War and Justice League doing it), but it's very much prevalent here - whilst some of the main characters get a ton of development/screen time, with a lot of them sorta just feel like they're tagging along for the ride and don't really contribute to the plot in any meaningful way as individuals. Kai and Cole in particular are just sorta there, and whilst Jay and Zane have some cool lines every now and again they don't really evolve as characters that much throughout the story. Shame really, since I was really hoping that the movie would make an attempt at not making Cole the "sorta-here-I-guess" ninja like he is in the show. Guess it simply wasn't to be.
Remember how I said most of the voice acting is pretty good? Well, there's an exception to that general rule, but there's a chance you may not have heard it - instead of having Good Morning America hosts for the GMN segments, here in the UK they replaced 'em with Ben Shephard and Kate Garraway from Good Morning Britain because localisation or something. Whilst Kate's performance is completely fine, Ben's is... to put it blunt, pretty darn awful. I dunno whether it's the way he's saying the lines or how it sounds like they're not written with his voice in mind (I wouldn't be surprised if he's got the same lines as the US GMN hosts, but I've got no way of checking that), but either way it sounds cringey as hell and I don't like it no siree.
Without going into too many details, the framing device the film uses serves no real purpose plot-wise and really doesn't need to be there honestly. They could've completely cut it and it wouldn't have made a shred o' difference.
Warner Bros really, really shouldn't have revealed Meowthra in the SDCC trailer honestly. If they'd kept it secret until the film's release as a right-outta-left-field reveal I really would've had more respect for the film than I do now.
The climax also feels kinda... rushed? Like, the moral lessons happen and then the animated stuff just sorta... ends. Doesn't feel as earned as TLM or TLBM.
There's a couple of scenes near the beginning that sound awfully devoid of background sound/music, almost to a degree that it makes the thing feel like WB sent out an unfinished cut to cinemas. S' weird.
The meme-y ultimate weapon montages? Awful. Please don't do that again.
Occasionally the ninjas describe things as "So ninja!" and it feels very "how do you do fellow kids" each and every time.
Zane does not dab in the film.
All in all, if I had to rate the film I'd give it a firm 6/10 - it's not the absolute worst film I've seen this year (to be fair I had to sit through the bloody emoji movie so I wouldn't take that as much), but there's much better out there to watch and I'd encourage you to do so.
tl;dr: Save the money you'd spend on film tickets for the stunning sets and just wait 'til the movie arrives on DVD - it's competent but really not worth the cash honestly.