Coming into this series, you just see another lame Magical Girl series... but what caught my eyes was the studio that was making this series- Shaft. Seeing this, I decided to give this series a try... and I am very glad that I did. It took all of my preconceived notions about what a magical girl series is, and destroyed them. That being said, if you are to only watch one Magical Girl series ever, this is the one you should try.
This series is, in a word, dark. The ending, in a word, is bitter-sweet. This series manages to conceal the ending while making it painfully obvious. It manages to hide the main character while making her be extremely obvious. While leaving the viewer extremely satisfied, you're left wanting more. The ending hits on a somewhat philosophical note while not being as absurd as some endings (See Evangelion). As a whole, this will be one of those must watch series in the future.
This series bares a very distinct style of character design and a style of animation that, in itself, can be quantified to nothing other than Shaft. While having constant art shifts in a very well preformed method. If you can get past the original character designs, then there is no reason to complain- this blend of different mediums, smooth animation, and often detailed backgrounds is very well done.
Both the themes http://www.youtube.com/watch?v=WBYbBrnexOQ and http://www.youtube.com/watch?v=SlL1hMwLaZw (just the songs are the same) and the actual soundtrack are extremely well done. While blending in the ED into the sound track, they do an excellent job of intensifying the emotions felt in many of the scenes. The voice actresses themselves do a great job and are, for the most part, fairly well known, ranging from Aoi Yūki to Chiwa Saitō and Eri Kitamura.
The characters in this series manage to, in many cases, change dramatically, both in the series and simply in the viewer's eyes, while maintaining a sense of reality in them. Several of the characters are extremely well developed and almost all of them are very likable. None of the characters fall neatly into a trope which is very rare these days.
If you are going to choose just one series from the Winter 2010 season to watch, it needs to be this one. It was very well executed and I can say that the delay getting the last two episodes (caused partially by the earthquakes and tsunamis in Japan) made the wait terrible.