While watching this series I just kept feeling that there was something... missing that could easily make it better. After finally finishing it, what could make it better makes an appearance (probably what made the second season so much more popular than the first), but, by then, it was too late for this season to be redeemed.
What this series has: It tried its hand at comedy, fan service, action, and mecha battles, but none of this was quite satisfying. There was some character development, but so much of it was of the secondary characters that it seemed like a waste. There was mystery, but the majority of that was solved rather quickly, or ignored. It throws you right into the action and rarely slows down. But by the time you start to understand who is whom and the different factions, they bring out a new faction or a new person to an old faction, or a person you've already seen but looks COMPLETELY different because she isn't wearing thick glasses and long pig tails. It also has a lot of details of the background of this world filled with science, demons, magic, the church, and many other things, but it all feels rushed. The only thing this series seemed to have done correctly was lay a basis for a second series, which the last episode managed to do beautifully.
Is it worth watching?: Well, I haven't seen the second season yet, and that will hold the answer. The general opinion is that the second season is a much stronger season, and, if what the last episode had to offer is any indication, it probably will be. If so, then, yes it will be worth watching, even if it is only so you can understand a decent second season. If not, then, well, no. Not worth watching.
Edit: I have now seen the second season, so I can amend this. The second season was strong enough that this first season is worth watching. If you can get through the sub-par crazy happenings of the first season, then I would suggest attempting to do so, if only to see the second season. It had much of what was missing in the first season, and the most important thing that was missing is abundant. That being emotion and risk. The risk is finally given at the end of the season, unlocking a greater since of emotion from the characters and a better ability for them to connect with the viewer. This then allows there to be more tragedy portrayed and the entire series to actually be worth watching. That being said, only watch THIS season if you will be able to watch the second.
Non-direction based critiques: The OP/ED took a little while for me to get used to, but by the end of the series I began to enjoy them. The animation was average. The characters were... ok. Some where likable, but I didn't find any of them amazing.
After I see the second series it is likely I'll come back and edit the part about the second season, so there will likely be an update in 3 days or so, for all of those that will be patiently waiting (just me).