Hidan no Aria or Aria the Scarlet Ammo is hardly an anime that could be judged by its name or by the genres mentioned here. To put it straight the anime does have action and comedy and school but its hardly the main part of it, its all about girls and guns (and swords) and the cliched over-used recycled gibberish scenarios (hence the G-shock). As a matter of fact, neither girls nor guns nor both put together made the anime any good but a lot of us (including me) are sucker for these types and thus the popularity.

The plot is set in a school that grants special students permission to take up missions and engage in combat using arms and ammunition. So you get talented snipers, gunmen, sword masters and special ability users produced by the school. Our protagonist, Tooyama Kinji, after a sad incident has decided to quit the combat division and live a normal life when suddenly he gets involved with a killing incident, a girl falls from sky, and activates his 'Ero-super-mode/ Hysteria mode' by contact with the flat-breasted Aria to set off the new course for his life.. so you can see :
1-sudden accident 2-girl falls from sky(special girl=cloud these days) 3-perversion=power and 4-rescue heroine :all 100% checked for predictability.
After being persistently sought after for his Hysteria mode (activated by ecchi stimulation and heightens his senses out of his urge to protect females) he decides to help Aria absolve her mother from false accusations.

The protagonist is surrounded by a flat chested tsundere, a plump yandere childhood friend and a femme fatale as a starting dose to his harem, hopefully the other female encounters will also enter into harem cruise if they dare to make a sequel. The plot throughout is quite predictable and strewn with your usual harem scenarios, depth and mystery are hardly much of a factor and most scenarios are mediocre or dumb or both. Therefore, as an intellectual facade to give a perception of depth and mystery they decided to use the offsprings of famous characters- Holmes IV, Lupin IV, Jeanne de Arc (was it 38th ?) and the Dracula himself who was generously available due to his immortality. Except that Holmes IV has more intution than deduction abilities, Lupin IV has to use others to steal, Joan of Arc 38th can use ice and of course! all of them are girls. By the way, Dracula is a 'stupid cur with black-and-white fur' (a werewolf like creature with white spots on his otherwise brown skin to mark 3 of his 4 weak spots --FYI that's so you can kill him easily). If they thought that they could create a masterpiece by stuffing something from everywhere they couldn't have been more wrong.
Seriously, the story could hardly have been good when the basis itself is flawed. I regard the story as mediocre and generously award it a 6/10.

[spoiler.. on a side note the last episode really pissed me off; Vlad, the EU no.2, great villain, is actually a dumb wolf, defeated in less than one episode. At last, at long last they f***ing ruined their own credibility.]

The animation is not so good. The character sketches are good for the most part, the coloring scheme is decent. But, the animation fares poorly, for one the scenes are not so fluid, they seem cut-off sometimes and another the angles are definitely not the best they could have given. I don't think spent much in the animation department cause although the guns and characters are well made the action scenes don't come out as great. The backgrounds aren't spectacular either.

The sound department is fairly better than the others. I don't have sharp ears to analyze the quality of voice-acting and BGM and grade it qualitatively but I consider most of the voice acting as good and the BGM nice though not spectacular. The OP seemed good to me, the ED was good and had a great accompanying animated scene.

The character zone is also starved of stagnation and ordinary means. Kinji, the male lead is a retarded and weak student with a hidden potential due to which he is either coveted by or recklessly thrown in the midst of dangerous women. A masculine instinct to protect females gives him the strength to survive. The female leads include a flat-chested with the greatest potential to sexually excite the hero, a yandere childhood friend madly in love with him and a dangerous seductress bent upon using him for her own devious schemes. The rest of the characters are also derived and hardly have a hint of originality. The development also follows the predictable route to a classic harem. Well, since Aria plays a greater in the plot, her development is comparably better.

You can see how much of a hypocrite I am since, despite all the scorn and cynicism, I still gave the enjoyment a 7/10. That's because, even though it's full of shortcomings, I still like the anime because of its comedy-harem nature (not that i watch every harem but I do watch a lot of them).

As an overall opinion, if you're into it because of mystery or action or something new, I suggest you drop it and watch better ones. On the other hand, die hard fans of harem are always welcome.. come friends, no matter what happens if it has an H its a Must-Have, if its a Harem its a must watch.