It rectified the two main gripes I had with TFA that somewhat restrained my enthusiasm for it. First, the pacing was appropriate to the story. Second, it wasn't marred the entire time by the fact that it's the first third of an elaborate mystery master plan. It kind of spoiled the fun for me in TFA a bit that we got so many little hints at something that we know will be resolved later, but boo hiss, no spoilers! Put differently, Rogue One wasn't there to set up another film, although technically, that's what it actually did.
I also can't believe that they actually managed to tell the back story of ANH without ruining any of it - but not only that, they made ANH better that way! Seeing the massive sacrifice that was necessary just to get those plans significantly raises the stakes of the events in ANH. Moreover, it left me with a sense of awe, seeing the large cost of a small victory. It was a good thing for the film that all the characters died. We got to know them as human beings and got emotionally invested in their lives and struggles, and their sacrifices are tragedies. This way, the Empire actually appears as evil. Not just a juggernaut Goliath to fight, but a power that will remorselessly kill and destroy people dear to us - something that never actually occurred in the Original Trilogy.
I loved seeing Vader's castle. Some old descriptions of the way Lucas envisioned it are floating around the internet, and this is exactly how I imagined it. I am also glad that the film used all the references to the OT sparingly and convincingly. It would have been unbelievable to not have had any callbacks to ANH, since this is a direct prequel, but I'm glad that it was done in ways that actually served the plot. I had fear that the Death Star would blow up some other planet before Alderaan, that Vader would lead the final assault and fight the heroes with his light sabre, and midway through the film I expected to hear that Jedha would turn out to be Tattooine... and none of that happened.
I was impressed by CGI Tarkin. It did become obvious that he was CGI, but it would have taken me a while to see hadn't I known that Peter Cushing is long gone. It's a fascinating document of how far CGI has come since the Prequel Trilogy. Seeing Leia in the final shot was weird, since I had just heard three hours earlier that Carrie Fisher has died, but I think the scene was great. The whole finale with Vader going after the disc was. And it even gives a satisfactory explanation to why they didn't just duplicate the plans and sent them out via Rebellion intranet.