If you like Star Wars, it's a must-see. If you like space action movies, you should watch it. If you're at a loose end and the kids are mithering, take 'em to the cinema - it'll knock their socks off. Don't go in expecting multi-layered narratives and nuanced characterisation, Rogue One is a disciplined journey from A to B. It's a tightly plotted, entertaining romp with no loose endings, and works fantastically well as the immediate prequel to Episode IV.
Like many millions of people, I'm a fan of Star Wars. Not a superfan mind, I'm not into the expanded universe, the novels and comics or anything like that. After all, since Disney prised Star Wars out of George Lucas's decadent grasp, the new overlords of the universe have declared the films canon, and that's about it. Not that I'm bothered. When I first saw the opening of A New Hope (then plain old Star Wars), you didn't need much to tell you who the goodies and baddies were. The Empire are transparently evil, the rebels unambiguously righteous. There are no greys. I grasped this when I was little, and so did the young 'un sat next to me at the cinema who kept saying "yes!" every time an X-Wing offed a Tie Fighter.
This simplicity is interesting because, ultimately, what makes the Empire 'evil'? What makes the Rebellion 'good'? With the Empire, it's easy. Darth Vader looks like a bad 'un, and within seconds of our introduction in the original film he's throttling a hapless rebel soldier. Palpatine's nefarious scheming sees his immersion in the Dark Side of the Force render him a rubber skinned monster. And in Rogue One, the Empire burnishes its evil creds with wanton acts of murderous destruction thanks to the newly operational Death Star. The main antagonist, Director Krennic is effectively a Nazi weapons scientist with Bond villain tendencies, up to and including the tendency to murder subordinates for little reason. Vader is the same, threatening Krennic with a sticky end if he fails him and the Emperor - a nod to the absurdity of The Empire Strikes Back where he kills off imperial officer after imperial officer for their failings. It strikes one that climbing the career hierarchy to earn the privilege of instant death working alongside the Lord of the Sith can't be a powerful motivator.
And therein lies the problem with the Empire. What is it about? Krennic is portrayed as an ambitious bureaucrat, so much so that Vader chides him for it. But there are no values, no matter how twisted motivating his murderous career path beyond personal advancement. And this is the same with Vader and the Emperor. For all their mystical twaddle about the Force and the power of the Dark Side, what's the point? It's power for power's sake. That's fine for individual motivations, but you can't carry a society on it. The expanded universe suggests the Empire has a whiff of Nazi-style human supremacism about it, but apart from their military personnel being either human clones or droids of some description, there is little in the films to suggest xenophobia and racism toward aliens.
Ditto for the Rebellion. We know from the prequels that they see themselves as heirs to the old republic brought down by Palpatine in Episode III, but apart from that, what are their values? Yes, they're against bad things. They don't like and are persecuted by the Empire's goons. Presumably they'd like to see democracy restored in the galaxy, but beyond that there is very little. They come over like many an earnest anti-capitalist activist. Very easy to identify what they stand against, difficult to pin down their alternative. The Force as practiced by dear old Obi-Wan and Luke Skywalker isn't much of a clue either. It emphasises the manipulation of the unseen forces that bind matter together, and Jedi training involves suppressing and rising above the passions through self-mastery. Selfishness, avarice, desire, jealousy, vengeance - all these are paths to the Dark Side. And, again, for what? To uphold galactic democracy and rescue kittens stuck in a trash compactor?
Where the Rebellion does noticeably differ from the Empire is with the integration of alien races. This theme continues in Rogue One with the Mon Calamari (of Admiral Ackbar fame) spearheading the Rebel assault at the end of the film. And throughout the saga, aliens are integral to military operations (though, bizarrely, none get to pilot Rebel X-Wings, Y-Wings and what have you).
And so what we're left with are incredibly simplistic notions of evil and good. The Empire is the grim outcome of indulging the self to the exclusion of all else, and the Rebellion springs from abstinence and the rejection of this-worldly things. Which is ironic considering the cinematic celebration of these values have created the most bankable movie franchise in existence, and fed the gaping maw of profit-hungry production companies, cinema chains, and toy merchandisers for decades.