It worked because it wasn't rushed: the first comedic half wasn't a pointless filler, it did set up the story while getting you in the right mood, and the second half took all the time it needed to work on the drama without stereotyping it too much.
And while it had to sacrifice time travel plausibility to make the drama work, it did start from decently thought out premises, taking enough time to work upon those too so that science nerds (like me) would be happy enough about the effort to also be quite forgiving when later on many of these premises would be violated.
That's what makes it great and a must see for every fans of the series.
It is now up to Kurisu to find a way to get Rintarou back.
Source: Wikipedia It goes without saying, DON'T watch this until you watch the original anime.
The series sold well, they just had to crunch out something more, even though there was little genuine plot related reason to, nor themes or characters that really offered chances for further development (and, indeed, there's hardly any real development here).
On the other hand, they could have gone the usual, lame "recap gekijouban" way, but they still tried to offer something new, so kudos for that are in order nevertheless.
While in my opinion quite overrated, mostly due to the shameful average level of the competition really, I can summarize the roots of its deserved success in one word: balance.
As I've thoroughly explained in my related review, while it tried to tackle more than it could handle when it comes to time travel plausibility, stumbling on worse and wider plot holes as the story went on and became more serious, the wise choice of first presenting you with a good comedy and only later on switching to drama did the trick, as it gave you the time to grow enough fondness for the quite charismatic characters to be able to really enjoy the following drama even if it came with a - mostly - inevitable crumbling of all the fictional physics rules about time travel.
Thus, while the drama per se isn't actually all that bad (although it definitely is a bit too much of the been there, done that kind), without a charismatic lead to pull you in everything becomes if not really boring at least quite... Neither there is any time (nor effort, actually) to give some space to the rest of cast either: they orbit around to be plot devices in a couple instances, delivering a key sentence here and there to move things, but they mostly are mere extras.
Which, again, is to some extent justifiable given the time constraints of the movie format, but still contributes at making the product feel overall quite shallow in comparison to its prequel.
Moreover, Okabe is chained into a side role this time around, the leading character, the one through whose eyes we are told the events being Kurisu.
And, well, as said, without Okabe as a filter she like everyone else is a rather dull, stereotyped character.
As for technical matters, the movie feels very consistent with the series.