Witch Hunter Robin is one of the best anime shows that I have ever seen (second only to Gunslinger Girl). There is literally nothing about this show that is bad. Everything, from the animation to the voice acting to the music is absolutely superb.
I'll start with the main story of the series, and I'll steer away from too many spoilers. The series revolves around Robin, the newest Witch Hunter at the STN-J. The STN-J is the Japanese branch of an organization called Solomon, a worldwide organization that hunts witches. The first half of the series takes a "witch-of-the-week" approach, but it really works for WHR. This first half allows the viewers to get to know all of the characters on a normal day-to-day basis before diving into the main plot halfway through the series. Again, the main plot focuses on Robin and her discovery of what she truly is. The final half of the series is pretty serialized, and it works excellently. There's enough developments to keep people hooked from episode to episode, but it never loses that touch that made it feel so good in the first half of the series.
Now onto the voice acting. I've only seen the dub of this series all the way through, but it's excellent. Kari Wahlgren proves that she's one of the best voice actresses in the business with her excellent portrayal of Robin. Wahlgren's voice is very unique from character to character, and it shows in this series. Crispin Freeman is excellent as Amon, the cold in-the-know guy with a softer interior that is not seen until later in the series. Johnny Yong Bosch is great as the brash Haruto Sakaki, and Dave Wittenberg is excellent as the computer hacker Michael. Every actor in this series excels at their part, and there are no standout roles.
The animation is also superb. Witch Hunter Robin is a dark and creepy show, and this is reflected in the animation scheme. The schemes are very dark and they're very realistic. It creates this sense of creepy realism that pulls you into this other world fully believing that it's the real world. There are outstanding locations, particularly the STN-J and Harry's. The character schemes are also very unique and different, so you'll never confuse any characters.
The music is one of the standout achievements of this show. Taku Iwasaki is an outstanding composer, and his music for Witch Hunter Robin is nothing less than amazing. He has many different and unique action pieces, the best clearly being "Flame", Robin's attack theme. The piece sounds like a mix of Latin and Irish music blended into a stunning and beautiful sound. WHR is also great with the slower pieces. There are several piano solos in the series (Harry's, Open Your Eyes, and Allay Pain, the piano version of half-pain), and they're all stunning and excellent. Each one is unique and different, and each one evokes a different feeling from the listener. Outstanding music.
All in all, Witch Hunter Robin is simply one of the best. It's got a great plotline, excellent dub voice acting, amazing animation, and it has some of the best instrumental music I've ever heard in an anime show. This is not a show to miss. Go out and buy the Anime Legends set now!