I'm a huge fan of "Puella Magi Madoka Magica." Like, a really huge fan. As in, it's-in-my-top-three-favorite-anime-ever huge (for the record, the other two are Fullmetal Alchemist (the 2003 version) and the movie Paprika). While I haven't seen either of the first two movies that retell the show--because the show was fine as it was and those movies just reek of "cash-grab" to me--I have watched through the show's twelve episodes at least six times by now and I'm pretty sure I'll end up watching them again a few more times.
This was a series that brilliantly deconstructed the "magical girl" genre, though even disregarding that and taking it on its own merits, it has an interesting story, beautiful animation blending numerous styles, a fantastic soundtrack--though Yuki Kajiura's work is always impressive--and flawless pacing (something that many, many anime struggle with). It's a heart-wrenching tragedy that nonetheless comes to an upbeat and satisfying conclusion. As a stand-alone story, it's near-perfect and I wouldn't change a thing.
And now we have a movie continuing after where the series left off. Which is really something that wasn't needed, for obvious reasons. That being said, "Rebellion" is really impressive. The animation is even better than the show, the soundtrack is just as good, and it does take the story in an interesting direction, even if it doesn't really make a whole lot of sense at first.
Let's break it all down then, and look at this movie one piece at a time. First, what it does well:

As stated above, the animation is absolutely gorgeous. Really smooth, really pretty, and the surreal environments throughout never stop being interesting and eye-catching. The fight scenes are directed particularly well.
The soundtrack is just as good here as it was in the show proper, with all of Kajiura's usual style, from the hard electric guitar riffs to the classical influence to the wailing choir. There's some really nice use of character leitmotifs from the show as well, often either arranged or combined in new ways. I'm a big fan of Kajiura's work, and both the show and this movie make good use of her talents.
The story is really cool up until the last twenty minutes, but I'll get into that later. Suffice to say, if you're a fan of the show, the movie will keep you just as entertained.

And now it's time to get into the less good bits:

Nagisa Momoe. What a pointless character. I mean, I get why she exists: Charlotte is popular among the fanbase so it makes sense to bring her back in a larger role, but she really contributes nothing to the overall plot. I'm not exaggerating when I say that you could remove her and you would have to change barely anything in the story, as Sayaka could easily fill her role here (and sort of does). However, Momoe is responsible for giving us the amazing Homura vs. Mami fight, one of the coolest fight scenes I've seen in any anime ever, so for me, all is effectively forgiven.
And now the big one, the thing that's made this movie so controversial among fans: the ending. Oh god, the ending. It's rare that you see an ending this divisive. I'm going to try my best to avoid spoilers here, but I'll throw in my two cents: I don't hate it. I don't particularly like the ending, but I don't hate it. I think the movie would have been fine had it ended 20 minutes earlier, but I also think that Homura's actions do feel logical and natural for her character. It could have been foreshadowed better (it is foreshadowed, but very, very poorly), and there could have been a lot more buildup, but it's not as awful as some people are making it out to be, in my opinion.

The big problem with this movie is that it really doesn't work as an ending to the series. Overall, it works fine as a continuation, and I think it's definitely worth seeing, but as an ending, it's far weaker than the ending of the show. Hopefully they'll continue to expand the Madoka Magica universe--and with how successful both the show and movies have been, I predict they probably will--and give us more of the elements that make it so unique and enjoyable. Maybe next time they'll do more with Momoe and actually give her character a point. Maybe next time they'll expand on the twist ending here and help justify it better.
As someone with a lot of love for this series, I can hope. After all, hope is what Madoka gave to the world.