I won't bother putting in the synopsis for both the original Guilty Crown anime and this, uh, 'summary' of the first eleven episodes, as it can be easily read from the Guilty Crown anime listing.
Like the review before this, the overall rating I've given for Kiseki is based purely upon sound and art, which doesn't really differ from the main anime, as it's all taken out of the main anime anyway. The animation and soundtrack alone are warrant a recommendation for the actual 22 episode Guilty Crown anime, which can be enjoyed even if you don't find the story or the characters overly that interesting.
Unless, you're like me, and did find both extremely enjoyable parts of the anime, then it totally just becomes kick-ass.
So this attempt of a summation is, at best, only really worth viewing if you want a really quick recap after not having seen the first half in a long time, or if haven't seen the first eleven episodes. And even then, it shouldn't even be considered as a substitute; it only makes watching this confusing, as opposed to just frustrating.
Even after viewing Kiseki, the summation that they decided to leave you with is extremely short-handed in the sense that you probably won't understand the main story, most of the more minor, but equally important, parts of the story have been left out in order to cram it all in under an hour. To save yourself the confusion, and to experience the awesomeness that is Guilty Crown, I highly suggest just watching the eleven episodes.
Coming from a biased viewpoint, where I thoroughly enjoyed Guilty Crown, it would make sense that I would recommend the actual anime either way, and of course I do. But even then, when I suggest not to watch this, it's not just because the recap doesn't do the anime justice, it's just really that terrible.
TL;DR: If you feel as if you can't sit through the first episodes, whether it be for the first time, or as a refresher, then it does it's job at putting across the bare essential knowledge from the first half. My suggestion, however, is just to watch the first half again.
If you are ever in a situation where you need to explain something that hasn't been done justice, this should be your go-to example. I have a large problem with Kiseki, because Guilty Crown is my favorite anime. The issue is that Guilty Crown is a 22 episode long anime with lots of character development, great art, an absolutely phenomenal score, and a very interesting and compelling storyline. Kiseki brings only some of what makes Guilty Crown such a great anime to the table. By condensing the storyline, you lose out on soooo much information, making it difficult to comprehend and thereby making the enjoyment
By shortening this, it all seems really all over the place and you don't really understand what's happening unless you've seen the anime, sometimes completely removing gigantic plot elements and leaving you in the dark and wondering what in the heck is going on. Kiseki tries to combat this with a narrative, but in the end that tells you very little and slightly breaks immersion. The narrative is not only an odd design choice, but it LIES to you, claiming things that, if you had seen the actual anime, you would instantly realize as wrong.
Kiseki is also a waste of time for the art reasons. Even if you are a huge fan of GC like I am, do not watch this. Just watch the actual anime. The scenes they used are copy and pasted from the original piece, and therefore invalidates Kiseki as a potential way of entertaining yourself even more.
Kiseki completely omits even saying some characters NAMES for the longest of times. Some of these characters being very important, sometimes pivotal, such as Ayase. You wont be able to understand why a character is important or even what they're doing unless you had seen the original.
I give Kiseki a four out of ten for great animation and sound ALONE, because the storyline is non-existent. I can see no reason to watch Kiseki, because if you're actually interested in the anime, you could and should watch the whole thing for free on Hulu, or just buy the dvd. This is a short review because all that needs to be said is that Guilty Crown is great, and I highly suggest it, but this is not the way to watch it.