It's a mixed bag.
The start is extremely disjointed, with lots of rapid cuts to different planets and a lot of unnecessary exposition (yes, we know the Imperials are building a planet-killer). The two main leads (Jyn and...whoever the other guy was called) are unlikable and poorly performed, and they get a very ham-handed romance in the last half that comes out of nowhere and just looks...eugh. At least it's better than Anakin and Padme.
The first segment of the film is, honestly, kind of a waste of time. Saw Gerrera shows up, is cool for a while, then does nothing. In general, until the midpoint the narrative is rocky, the special effects suck, the dialogue is crappy and laden with exposition (just to make sure that you know that there's a Death Star, but - get this - it has a weakness!!!), there's one incredibly forced cameo that makes no sense in retrospect, and it all turns out to be pointless.
The few high points during the first half were a handful of lines from the droid, Donnie Yen's introduction (all of Donnie Yen, basically), and one or two key scenes that establish that the Rebel Alliance are not all saints and martyrs. In the end, though, the moral ambiguity just does nothing but make me stop caring; the unscrupulous Rebel general pursues a plan that - without spoiling anything - makes no sense in context, and what should have been a mid-film reveal is instead literally stated out aloud by two characters at two separate points in the film.
Then it hits the mid-way point, and it starts to get better. The special effects make a huge jump in quality, leading me to think the earlier segments were mostly reshoots. Donnie Yen and his buddy are great. There's a gorgeous destruction scene involving the Death Star that really deserves a medal. Then the action hits Scarif, and it basically gets great. It's an extended, forty-minute-long war sequence, essentially. You've got AT-ATs, starfighters, star destroyers, there's lasers everywhere, everything explodes - it's great. You can see that this is where the work got put in. The film's central conflict ends nicely, the villain is defeated by irony, everyone gets a chance to shine during the long ensemble action sequences, Scarif is just beautiful, and I don't want to spoil anything but it's sad, and...
...then there's an incredibly forced coda, complete with CGI Carrie Fisher, which exists solely to tie the film in to A New Hope down to the establishing shot. It's awful. It also makes no sense; it shows Bail Organa saying "I'm going to go to Alderaan, wink wink nudge nudge, I know a girl," there's a shot of R2-D2 and C-3PO on Yavin, and then...for some reason Leia and her ship, complete with two droids, are on Scarif later to receive the Death Star plans and flee into hyperspace. It doesn't make any goddamn sense; why would you bring a protocol droid on a warship, why would Leia - a politician - be on the ship and at the battle, why wouldn't she be at Alderaan where we were just told she was, if it was a warship why did Leia insist it was a diplomatic vessel in A New Hope, just...very forced. A disappointing way to end the film.
On cameos: Grand Moff Tarkin makes an extended appearance that some reviewers said was very jarring, because he's being played by CGI Peter Cushing. The CGI is of extremely high quality, but the uncanny valley is still very noticeable. Weirdly, it didn't bother me that much, and when I thought about it I realised why; Tarkin is meant to be creepy and unsettling. Having CGI!Tarkin slam face-first into the uncanny valley works, because it creeps you out to look at the guy! He doesn't look human!
Vader gets a couple of cameos; one in the middle, which is done very well, and one in the end, which literally gave me an erection. He gets to show off, is what I'm saying. He's ultimately tangential to the plot - I was kind of expecting him to have more weight, but maybe they're saving that for a Vader standalone - but he does get to show us all why every Jedi in the galaxy was pissing their pants at the mere mention of this guy's name.
As a final note; if you didn't like TFA because of Daisy Ridley or whatever, blah blah affirmitative action blah Mary Sues blah blah, you won't like Jyn Erso. While I wasn't bothered at all by Ridley's performance in TFA because I found her endearingly earnest, Felicity Jones just really rubs me the wrong way. She's the focus of the plot, and I don't understand why. She has no character development, her sole motivation is finding her father - who similarly gets very little characterisation beyond being a walking plot device - and right at the end of the film she turns into a nigh-suicidal idealist with no real reason other than because the film needs someone to stand up and say something inspiring. All this when any sane person who went through her story would feel disillusioned and betrayed. Her dialogue is awful, both in content and in delivery, and until the climax on Scarif, I literally didn't care about her at all.
But the climax on Scarif is great, some of the visuals - mostly destruction sequences - are wonderful, and the side characters steal the show more than once. I started out unimpressed, then actively peeved, but the last half was a steady improvement leading to a great climax. Without the Leia cameo, it would've been a wonderful way to end the film on a high note.
