If you hate inconsistencies, stupid plot devices, a horribly flawed setting, and poor characterization, Senki Zesshou Symphogear is your worst nightmare.
The start of the series is deceptively good, we see a concert that's animated brilliantly, and we have a seemingly gritty story; we witness Kanade die in the past, and Miku crying over Hibiki's grave in the future. This image the anime painted itself in the first episode is one of the greatest deceptions I've witnessed in any form of media.
After the first episode, the animation quality drops dramatically, at times you wonder how the producers thought they could get away with it. Epitomizing this is a scene as early as episode 3 where Tsubasa is walking along the corridor with her manager, which looks like a GIF pasted on top of a moving background. It's obvious that they spent most of their budget on the first episode, leaving them to cut back as much as they could throughout the rest of the series. By the end they managed to get a more acceptable level of quality, which was consistent for the rest of the series, so happily it manages to avoid totally failing in the art area.
The soundtrack is quite enjoyable, and goes some way to make up for the animation. The songs they sing as they fight are relatively good, and it can bring an atmosphere to fights that is otherwise missing. The Opening is fine and fits the series very well, but I personally preferred the ED, "Meteor Light" by Takagaki Ayahi. It took a while for it to grow on me, but it's a track that sounds dark and traumatic, which fits the atmosphere the series was originally aiming for.
The characters are terrible, and the relationships they share change for the most bizarre of reasons. There's an instance where a character hates another one so much she's tried to kill her, but her hate is instantly forgotten when the other character points out her room is messy. Most of the characters are characterized very poorly, and the protagonist is very annoying. However, whilst this is the case, most of the cast is likable, and so you'll find yourself liking most of them, even as they go through the harsh character changes that the plot forced upon them.
Unsurprisingly, the plot is also lacking. It's a very simple plot, the one plot twist the series has is so obvious that you can identify it 5 episode before. It's simplicity works for Symphogear's though, as it manages to avoid being too disastrous throughout most the series. However, in the final few episodes, it gets so ridiculous that you will be amazed. Stupidity like this is something that is rare, and therefore something to be treasured, as it will make you re-evaluate what you consider truly preposterous.
The setting is also so tremendously flawed that whilst you can happily ignore it if you so wish, even minor probing will expose questions to which there are no answers. For example, with the frequent attacks by the Noise, which you cannot defend yourself against and kill everyone in the areas they attack, why do people go outside so casually? Much as I like the outdoors, I can't hep but think that I like living more. Also, much as I love my current house, if there was constantly the threat of death, I think I'd move.
The inconsistencies are fairly easy to ignore, but if you think about anything in the series, you're likely to notice some. Take the Swan songs, they used to mean certain death, yet past Kanade's death, it stops being fatal, and towards the very end, there is no visible affect at all. This is of course because it was a plot device, but the unashamed way in which the anime casually allows these is dreadful.
With this all in mind, this is a very bad anime. However, there is one thing that works in Symphogear's favour that saves it from the worst, and that is that it's earnest in it's stupidity. It isn't trying to be taken seriously, and it isn't pretentious, which means it is very hard to hate. The characters that I hated at the start of the series grew on me, to the point I liked almost all of them. Take the girl who's lines almost solely consist of failed meta-comedy attempts like "If this were an anime" which infuriated me at the start, towards the end I almost found her habits endearing.
It's a strange quality of an anime like Symphogear, which is terrible, but still enjoyable to the extent where I looked forward to the last few episodes, to see if the episodes could outdo themselves in their silliness, but more importantly fun.
So, if you are happy to ignore all that is wrong with this anime, then you will be able to enjoy it. Despite the amount that is wrong with this, it's all surprisingly easy to ignore. However, if you cannot ignore the technical details in anime, then avoid this like the plague, unless you wish to see how badly executed an anime can be.