It was so-so. Nothing special, but just passing the entertaining-grade, which is all I was expecting from this series.
The setting was fairly original and it had a surprisingly good backstory, compared to most shows in this genre (like Shakugan no Shana). They actually explained how the ESP powers, the magic, and the whole academic city worked.
The action wasn't epic and jaw-dropping in any way, but it was enough to keep me somewhat entertained. The story does try to get a bit serious, but never goes as far as making it painful for a fairly shallow genre like this.
Another plus, unlike similar shows like Shakugan no Shana, in which I disliked almost every single character, they did a good job here making every character (save the two mentioned later) being at least acceptable. The only downside in the way the characters were distributed, was that the character this anime is supposedly about (Index) wasn't among my favorites. She has her charm, in her own oblivious kind of way, but I liked Mikoto a lot better. Fortunately, she had her own arc and got a decent amount of screentime. I can see why some people disliked a couple of the side characters, but they didn't bother me. In fact, I'd even say they were quite good for the role they had to fullfil, a shounen-genre like one.
The quality of the story is pretty much equal all the way from the beginning until it hits the final arc, which was were things got bad. It was completely stuffed with cliche's and introduced the only two incredibly irritating characters in the entire show, great way to get the fans warmed up for a second season, right? While the show is very much arc-based, thus not leaving you with an completely unfulfilled feeling at the end, it does shove the intention of a second season in your face, and not in a very appealing way.
The animation quality was good, on par with other 2008/2009 works, nothing more, nothing less. As for the sound, I couldn't care less. The theme songs weren't memorable enough for me to recall, even this short after watching it, and the use of music through the series was also pretty average and sadly, forgetable.
Overall, it was somewhat entertaining for a show this average. I'd suggest it to fans of action shows with bits of comedy, a tiny bit of ecchi and a grain of shounen thrown in, but there are shows out there in this genre that are way better.
Despite the whole feeling of "average" this show has, it really warmed me up for To Aru Kagaku no Railgun, the sequel / spinoff, having Mikoto with her over-affectionate kohai Kuroko in the lead, the two most awesome characters in this series.