Literature Too much wasted potential (Volumes 1 - 22, anime)
I came into this series expecting a Nasuverse-esque setting; a highly-detailed constructed world with lovable, complex characters. To sum it up in a short phrase, it's a hit and a miss.
Toaru Majutsu no Index has so much potential in its setting and plot, but settles for a mostly mediocre execution.
The main problem with this series is that it introduces so many characters, but fails to develop a good number of them. The most glaring example is the eponymous character, Index, who becomes nothing but an open target in the later novels. The fifteenth novel introduces so many factions and characters who just get thrown away for the sake of plot. And worst of all, some of the character development just feels out-of-place and unnatural; the direction that their characters take is just jarring compared to their original personalities (Accelerator and Mugino are the most glaring examples).
The narrative also leaves much to be desired. When the narrative settles for jarring descriptions of the female body interspersed with action scenes, or when the characters just dump on you pages of exposition, you know there's something wrong.
This is a novel that, I believe, is better suited to an animated format, but unfortunately, the problems which permeate the novel get translated into a different form in anime. The anime favors fanservice and cutting down/downplaying much of the interaction between characters. The first season did not have too many glaring problems, but the second season somehow got worse in terms of direction.
Not to say that there aren't any good points; like I said, it has a rich setting with diverse characters and interesting use of superpowers, which is why it's all the more unfortunate that it doesn't settle to become better than what it is right now.
Index, in my opinion, is great if you want to just sit back and enjoy sci-fi/urban fantasy action. However, if you expect it making full use of its setting and characters, or if you expect it as an example of a great story, then you will only be disappointed.
There is, however, some hope. The New Testament novels seem to fix some of the more glaring problems with its story, and there is still hope for JC Staff animating the remaining novels in a better manner. Until then, these problems will stay with Index.
Literature Magic, Espers, and crazy tech.
I highly recommend this work to anyone who enjoys fiction with a lot of powerful characters and crazy plots. The magic system is refreshingly different from all the others I've seen - everything is based off of some legend or religious story, which guides it away from being the normal beam/elemental spam magic, and includes some awesome bonuses for those who know their myths. All the fights against power users, whether magic or otherwise, are won by analyzing the opponent's power and finding its weak points, so the fights are not only spectacular but intellectually interesting.
Now for my big complaints about the series. If you were hoping for epic face-offs between espers and magicians, you're going to be disappointed most of the time. Excepting Kamijou and Motoharu, espers fight espers and magi fight magi. (And these characters have some of the blandest powers in the work.) Even at the end of the light novels, there is very little hostile interaction between the two sides.
As for the other complaint? Most of the harem stuff, but especially the 'outfits' and 'power levels'. The bothersome part is that they treat it almost as seriously as they treat the fights, and it's not funny most of the time. Sorry, but it's hard to take sneaking into a secret base seriously when the girl is trying to slip and 'accidentally' show the hero her panties (or just outright come on to him) every seven lines. The real humor killer is that it's monotonous without becoming a running gag. You just know what's going to happen when this guy meets a girl.
The main hero, Kamijou, although he's fun to watch in action, experiences little growth during the course of the series. (Not even a Shonen Upgrade.) As a secondary protagonist, Accelerator makes up for this by experiencing plenty of development, which includes being 'game-balanced' so he can participate in fights without being a Boring Invincible Hero. And then, after being crippled, there's the time he uses a shotgun as a crutch... The few times girls actually comes on to him, it's hilarious, because it's so rare. (For good reason, may I add. The guy's amazingly violent and unbalanced. IIRC, one girl was drunk and the other fell in love with his carnage.)
In short, it's a cool work with a unique concept. Go read it. (Or watch it, if that's your thing.)
Literature My Favorite Series
To Aru Majutsu No Index is flat out my favorite series for a few reasons. The setting is very unique; it's an Urban Fantasy that take place in a Twenty Minutes Into The Future world (far enough that it's stated there have been 3 Hispanic presidents of the United States) that takes place mostly (at first) in a city called Academy City that's 30 years more technologically advanced than the rest of the world.
In this world there are two ways to gain supernatural powers. The first is to become an esper by going through the Power Curricular Program in Academy City which gives you a single power level 0-5, where a level 0 has so little power it's not even visible to the naked eye and the seven level 5's have the power to destroy entire city blocks. The second is through magic where you turn your soul into mana, and use magic based off of stories and legends which can break the laws of physics and get really really powerful. It's impossible for a person to be both a magician and an esper at the same time or else the hybrid will self destruct.
The main character of the series is Kamijou Touma, who wields the Imagine Breaker in his right hand, despite being labeled as a level 0 by Academy City. His power can negate both kinds of supernatural powers, at the cost of negating his own good luck. He gets drawn into the secret world of magic after a nun named Index lands on his balcony, who claims to have 103,000 grimoires memorized and that she is being chased by hostile magicians.
From there the story expands all over the world showing the various factions of the world of magic, and the darkness buried deep in Academy City.
Eventually two new protagonists are introduced; Accelerator Academy City's strongest esper who is seeking redemption after murdering more than 10,000 clones in an experiment, and Hamazura Shiage an actual level 0 trying to survive in a world where people can destroy entire armies by themselves.
The series spans a light novel series currently 26 volumes long, two anime seasons, and a spin off manga based around Misaka Mikoto, which got it's own spin off anime.