Generally, direct prequels aren't great. They lack any surprise and they're full of fan-service references that mean next to nothing to the casual viewer. Rogue One suffers heavily with this problem.
However, it's not The Hobbit Trilogy bad. Rogue One is an okay film. But it doesn't live up to the promise. There was a promise of a unique visual style in the way it was shot, dropping the shackles of having to fit the style of the main saga. But it was bland and nothing special. There was a promise of interesting characters, a band of rebels each bringing something unique to the mix. But they were underdeveloped and most of them lacked screen-time. There was a promise of an exciting story linked to but different to the main saga. But it became about Father issues again, the strongest running theme in a galaxy far, far away.
All the little nods to characters from A New Hope were utterly pointless and stuck out like a sore thumb. It comes off like a fan film and that's not surprising since it's based on a sentence in an opening crawl. While it's nice to see Darth Vader (didn't know he was renting a room from Sauron in Mordor) he added nothing to the story, We shouldn't have seen anything from the Empire's point of view, we don't need to see the politics about the Death Star and how a commander is worried how he's not getting the credit. The Empire should have been a faceless, monstrous regime the Rebels were rebelling against. That way at least would have meant we could have avoided the CGI Tarkin that took me out of the film every time he was on screen.
The final act was interesting, felt dramatic and seeing characters die in a franchise led Hollywood film almost had some impact. Would have been more effective if the characters had been developed. K-2SO was great though and seeing him defending to the end hit me the hardest of all.
I still can't decide how I feel about the ending. At first I was like "Ah, so it's going straight into A New Hope." But then the film was like "LOOK!! IT'S A NEW HOPE!! YOU LOVE THIS, DON'T YOU? A NEW HOPE! IT'S THE SAME SHIP AND THEY'RE WEARING THE SAME OUTFITS! AREN'T WE CLEVER?! DARTH VADER! LEIA! A NEW HOPE!!" I think I know how I feel about the ending now. I hate it.
If these "A Star Wars Story" films are all prequels then all imagination is lost in one of the most imaginative franchises of all time.