The story has been done before, but that doesn't make it any less interesting. The way it folds out, slowly, and so that when you think you know what's going to happen next, things take a completely different turn is brilliant. There were some points, especially near the end where I found that things got a little complicated. There were plot twists that personally, I think the anime could have gone without. The ending was, well...awful, to put it bluntly. It didn't take away from my overall impression of the anime, though, which I suppose is good. I watched all 26 episodes in 2 days, and not once did I find myself getting bored. If it weren't for the crappy ending, I would have given at least a 9.5/10.
Beautiful colours, fluid movements and brilliant character designs. The animation of Mirai Nikki, to put it simply, was amazing, and probably my favourite thing about the whole anime. It managed to be different without looking odd, something a lot of series seem to struggle with. The backgrounds were detailed and drawn beautifully, rather than a lot of animes where the backgrounds are nothing but brief watercolour scenes. There were a lot of bloody scenes, none of which were over done. There were no characters getting limbs cut off, losing more blood than they probably have and still living. The injuries were realistic, and so were the attackers. I have nothing to complain about here.
I tend to skip openings and closings, but I didn't when watching Mirai Nikki. The opening set the tone for the show wonderfully, while the ending of each episode led perfectly into the closing. Even now, months after finishing the show, I still get the songs stuck in my head on a daily basis. The backgriund music wasn't as impressive, but I still loved it.
Unique, and for the most part, intelligent characters, complete with flawless designs--what more can you really ask for? Every one of the characters developed in their own way, making for a perfect cast. They all had their own personalities and quirks and as far as I could tell, weren't ripped off from anywhere.
I would rewatch this anime in a heartbeat. In fact, I think I will. If thrillers with plot twists and crazy characters are your thing, you'll love Mirai Nikki. Even if they aren't, give it a try anyway!