Totally okay movie, and actually pretty similar to the book. A lot of the details were obviously not included (as always when a book is filmed), and for that reason I think it really helps knowing the story and characters from before, although my friend who had no idea what was going to happen understood most of the plot.
The story was basically the same and the plot holes from the book were still there, with some added ones. For example one thing that was never explained was why the factions are secluded from eachother, and especially why the initiates don't get to meet their families etc. Actually they're just kept in seclusion to get used to their new factions and they do get to meet their parents again after a few weeks. After that they get jobs and sometimes socialize with other factions, but mostly keep to themselves to "keep the peace". Things like that were really sketchy and if I hadn't read the books I don't think the faction system would make sense to me at all.
The ending was also changed, with the
simulation being shut off by Jeanine rather than IT knowledgeable Four/Tobias and
them running away for seemingly no reason, whereas in the book they go to Amity to meet with the other Dauntless because their home quarters have been destroyed.
The most annoying thing for me though was that almost all male characters looked exactly the same which constantly made me confuse Al with Will and Peter. I literally thought they were just one guy, then two, then at the end was shocked at finding out there was a third one.