ChaoS;HEAd is a psychological anime with some action and a good plot behind it. Overall, it was pretty enjoyable, and something that is definitely worth watching; however, it isn't one of those extremely memorable anime like Death Note or Code Geass that everybody just loves. Actually, it doesn't reach those heights in the slightest, but that's not to say it was terrible either. Definitely watch it if you like short, psychological anime series, but if you're a casual anime watcher, ChaoS;HEAd should be watched after you've watched the Tops.

Story
Read the synopsis for more details. Basically, you have your generic "I hate human greed/selfishness and I'm going to destroy/remake everything to make a point" anime villain. Then you also have the generic male anime protagonist who attempts to save mankind. Sounds boring, but ChaoS;HEAd adds to this generic plot with its own unique recipes that makes the story worthwhile. There are unexplained points in the story for minor details that are never answered, but most are sci-fi related. It does make the story unsatisfying, but doesn't ruin the story.

Art
I watched ChaoS;HEAd subbed by m33w fansubs, which were 400 mb per episode, so the quality of the anime was great. Everything was sleek and clear. Besides that, the art style is also notable. It is drawn with seriousness in mind, so you won't be seeing any MOE characters or out-of-proportion body parts (besides the eyes, lol) that are common in comedy anime. The motions are very sleek and smooth, so you can just appreciate how many drawings came together to piece ChaoS;HEAd up. I haven't noticed any obvious reused background scenery, so that's a plus as well.

Sound
I'm not a big person on sound, unless it was noticeably awful. The sound in ChaoS;HEAd seems fine to me. The opening song really fit the anime. I personally didn't like the peaceful, pacific ending song, seeing as how it just didn't quite in with the psychological element of ChaoS;HEAd. Generally, the background music was pretty good and fitted with the various situations. The sound effects were satisfying as well. The effects for bodies .. "being damaged" were especially realistic *shudders*

Character
Characters were very unique in ChaoS;HEAd. Every main character has their own traits, from the queer Ayase to Yua with her extremely notable repetitions in speech. The protagonist, Takumi Nishijo, was especially interesting and quite different from generic characters, to say the least. His character development was also done very well. Most watchers will likely hate his personality at the beginning, and most will change their opinions ;)

Enjoyment
In the beginning, I didn't expect much. The first few episodes presented an interesting story and Nishijo's personality, albeit irritating, was noticeably different that most other anime characters. However, everything felt a little bland. But as I continue watching, it became quite enjoyable, as more characters were introduced, and more mysteries presented themselves, waiting for explanations in further episodes. The ending was especially good and is probably the highlight of this entire series.

Overall, ChaoS;HEAd was good, enjoyable, satisfying, but definitely not an epic/memorable anime. I recommend watching the first episode or two, and then take it from there: if you are interested, keep going and if not, hold it for later.