The first 'Puella Magi Madoka Magica' movie is a direct adaptation of the eponymous series, containing the events of the first eight episodes while leaving the last four to its sequel. Make no mistake, it is a very good movie. I just don't really see the point of turning a twelve-episode series into a two-installment movie series.
From its technical aspects, this movie is excellent. The soundtrack is fantastic, mostly using the excellent pieces from the series with a few new additions. The upgraded visuals are stunning, yet also very familiar; it's basically the same just with a higher budget (unlike, e.g., the Evangelion movies which used a bunch of questionable CGI in addition to the series' cel animation). The direction is also superb, as it already was in the series.
From its contents, well, it's just the first eight episodes of the series, streamlining a few parts. I wouldn't say that anything important was left out, though in some scenes I felt like the passing of time was undermined a little. I'm not perfectly sure why you would cut twelve episodes into two movies in an eight-four division instead of six-six, but I'm sure it'll make sense as soon as I've watched the second part.
That's the issue, though. The first movie is very much a first part. It ends with a cliffhanger, leaving pretty much all plot points dangling. That is not to say that the ending is unsatisfying or sudden; I would very much say that this is a very good point to stop, basically right after the big 'Holy cow!' revelation of the series (and thus probably the reason for the eight-four split). Just... what's the point of turning twelve episodes into two movies? To be honest, I'd rather have twelve episodes with upgraded visuals rather than two movies; it would be easier to portion, would reflect the passage of time better, and IMHO is just a more sensible approach to split up a long story.
I don't think there's much reason for watching both the series and the movies (except for experiencing the story twice), which I find a bit disappointing. Basically, you have to decide if about three-quarters of an hour of left out scenes are worth the downgrade in visuals; the overall story is basically identical. I just don't really see the point of this movie's existence. Still a good movie, though. It just doesn't deliver anything the series didn't already.