C³-bu, in a word, is confused. It doesn't know what it wants to be. Is it a slice-of-life about cute girls shooting cute guns? Is it a story about how the main character loses sight of what's really important while chasing a dream and how she works to gain it back? Is it a tale of the supernatural link that a girl has with an ancient Japanese warrior and how she uses her power to change the world? The answer is none of the above. It is instead a jumbled mishmash of all three. Due to their radically different themes they end up coming together like chocolate, steak, and curry in a blender.
The slice-of-life episodes have little to comment on. If you like cute girls and/or airsoft then watch them, they're well-done for what they are.
The supernatural element has only one major episode, number 4. It's brought up again twice near the end but never mentioned throughout the large majority of the series. The fact that they included the paranormal theme at all is mind-boggling. When they brought it up in episode 4, the watcher is led to believe that it will be the focus of the show. Yet it in fact makes absolutely no impact on the plot whatsoever. However, it's still done beautifully, and is in my opinion the best episode of the show.
The plot-driven episodes are quite good. Yura and Sonora (and to a lesser extent Rento and Rin) get some character development, and although the other characters are just there to fill out the cast and are underdeveloped it doesn't detract much from the plot progression. Although its end is less than fantastic they're worth a watch.
You may read this and think "This all sounds pretty good!" Well, that's because it is. Each individual element is good but they don't mix together very well. You may be wondering why all of these differing ambitions seem at odds with each other in the show. (Note: the following is speculation.) This rampant disorganization is likely due to C³-bu being Kawajiri Masayoshi's directorial debut. With no offense meant to the man, his filmography outside of C³-bu consists of only one lone entry - doing background art for Dantalian. However, the individual episode directors all have more experience than Kawajiri. Many have worked on very notable shows like Cowboy Bebop, Code Geass, Gurren Lagann, etc. etc. As his first time directing a series, especially with little other experience under his belt, one can see how he might have had trouble keeping reins on the episode directors and making it so that the whole series has overarching themes rather than just having them crop up and then disappearing.
As I see it, there are four ways to watch C³-bu.
1. Watch episodes 1-3, 5-6, and 12-13 for SoL with girls and airsoft.
2. Watch episodes 7-11 if you're more into plot and want a decent story of suffering and redemption.
3. Watch all of episode 4 and 17:00 to 20:35 of episode 11 if you want an interesting supernatural oneshot.
4. Watch everything from episode 1 to 13 if options 1, 2, and 3 all sound appealing. You can watch it for the entertainment value and appreciate it for what it is - a highly flawed show that was poorly directed, but has some legitimately good material despite its shortcomings.