Like many, many other manga, Gunslinger Girl has a brilliant premise but a halfhearted execution and inefficient delivery.
Yes, I do feel some pity for these assassins' destroyed childhoods, and I suppose their unrequited loves are very tragic and all, but it isn't written in a way that makes me sympathise. Maybe it's 'cos, unlike the target demographic, I need a better reason for my sympathy than "But the girls are so cute!" Gorgeous little girls or no, none of them seem to feel even a smidgen of remorse about the many, many people they capture, beat up, maim and murder; the only emotion most of them display is mindless devotion to their handler. I don't have a thing for "unconditional love" like most people do, and at any rate the handler-assassin relationship is way too power-skewed, awkward, lolicon-esque and forced for me to support. I don't at all mind reading about killers - especially since the people they kill are killers themselves - but the kids just never seem to worry about the dangerous, precarious nature of their current lives or what their futures will be like. In fact, they're disturbingly like normal girls, except that instead of playing sports after school they go to target practice, and instead of field trips they go out and gun down a bunch of terrorists. The bizarre incongruity between their actions and personalities within the organisation's premises and their actions outside of it means their situation never really 'hit' me.
Also, the portrayals are a bit too one-dimensional. Why aren't there any male assassins and/or female handlers? It would have been very interesting to see a parallel experiment being conducted wherein they used male children in similar situations and doctored them in the same way, to test out how a boy's higher endurance and strength panned out against their less docile and more willful, disobedient behaviour -- perhaps to see whether girls or boys are better suited for the job. And vice-versa for female handlers. How about a lesbian gunslinger girl, who struggles with the brainwashing that makes her adore her male handler vs. her true nature that makes her more inclined towards women, and how she finally makes peace with herself? A girl who's resistant to brainwashing and longs to escape, at the same time struggling with the knowledge that she's physically incapable of living outside? One of the girls meeting people from her past who recognise her and try to bring her back, only to be bribed/intimidated into silence by the organisation? Where are all those interesting, multi-dimensional scenarios?
But these "personal issues" of mine wouldn't have mattered if there had been enough plot and/or action to keep me engaged. The girls aren't involved in any of the planning, and the handlers themselves don't do much in the way of spying, intel-gathering or strategising. There are other people responsible for those things who work largely off-page, so we rarely - if ever - get to see the inner workings of the organisation, which is a shame as it would probably have been very interesting. The arcs are fairly self-contained despite the fact that several major antagonists are recurring characters, so there's no time to truly feel the stories of either the terrorists or their victims. (And I usually enjoy episodic series). There aren't any cool action scenes as such to keep you thoughtlessly entertained, so your mind strays toward philosophical reflection, but even that philosophical thirst is not satisfactorily quenched, so...
...It was a nice read, but I really struggled to get through all 15 volumes. But if you're a melting heart, love watching cute girls in action, and/or are a fan of forbidden/doomed/repressed romance, this is ideal for you.