I noticed that there seems to be a pretty stark contrast in opinions concerning this anime, so I decided that I'd throw my hat in the ring.
Story - 7/10
C starts out with a story which is pretty difficult to comprehend. The first episode of the show should leave you thoroughly confused, mainly because of this "deal", which is the primary focus of the show for the first couple of episodes. While the "deals" seem complex (with micro, mazzo and macro flations, and money somehow being involved in maintaing upkeep for skills, and the need to balance or bankrupt the end score with your opponent) it's actually really simple. It's pretty much Pokemon or probably a closer comparison would be Yugioh. If this sounds lame, it is. Numerous reviewers, including the official review for the site, have condemned this anime for just that. I find it difficult to condemn this anime for one aspect, which doesn't even make up most of its content. In reality these battles are important, but they're also secondary to the main plot, if you want to think about it in the terms of television shows think The Office or Parks and Recreation, these are shows which are about an office and a parks and reaction office, but they're more about the people working there. C is similar in that aspect and ultimately once the story really gets going, the show picks up exponentially and you don't even see many of these Yugioh-esque fights.
Animation - 7/10
The characters have cat-like eyes, that kind of bugged me for awhile, other than that it's pretty average. The fight scenes are nothing spectacular, nor are any of the character designs. It doesn't look exactly like every other anime in the world, with the same recycled faces, so I suppose that's a plus.
Sound - 8/10
The show's score is solid, so are the intros and outros for it. As for the voice acting, it's good, some of the characters are a bit bland, while others stick out. There are several "engrish" scenes, a lot of people give the show shit about this, but engrish is hilarious and I don't really see how you can't enjoy golden lines like: "As a rumor yes, I've heard of it. But no one's actually seen or heard of it."
Reviews you see complaining about average characters are probably right on the money here (haha, money, because it's about economics). I can't go into too much detail about some of these characters, as it might give important plot points away, but suffice it to say you've seen this main cast before (Mikuni, Yoga, Msyu). A lot of the secondary characters are pretty cool, Masakaki, Ebara, and the information broker are all pretty enjoyable. And I'm a sucker for romance, so in that the potential relationship that seems to develop between some of the characters is decent.
Overall - 7/10
This show certainly has its faults, I feel that the "deals", or fights which go on between the main characters is a definite con for this show, causing it to take awhile to really get started. Time spent on these monster fights could have been better spent building on the relationships between characters or simply developing the main character more, who we get very little background on. The anime is decent though, the ending is above average, however it will likely leave you with the same feeling as something like Angel Beats! or Eden of the East. Where a solid conclusion is given, but you feel that there is a much greater story, which goes on after the show, though it is a story we'll never get to see.