I've been meaning to watch the series - is it that good?
Don't feel like writing a 5 page essay on Gunslinger girl so I'll try and keep it short.
Season 1 of Gunslinger Girl had a very good base material but fell short in the end since they didn't explore the complex feelings that were part of the original manga version. Thus, it's 12 episodes of cute lolis with guns killing people. There is not much of a link in-between each episode either.
ALTHOUGH, they corrected all this in season 2 (Teatrino) which brings the two most important parts of the story that they had forgotten to put in the first season. First of all, they added the question of existence the little girls are feeling and also the feelings and motives of the foe they face.
Why were those 2 things important? That's because Gunslinger Girl is truly written like an Italian drama/opera. The author has always been a giant fan of those and wished to introduce this style to the Japanese audience. That is why he made Gunslinger Girl a tale of vengeance, bloodshed, impossible love and weakness.
Now, for anyone that wants to watch the anime, I'll suggest watching the first season since most of the character introductions are there. While it offers nothing really special, it is about the quality of the generic anime, which is still watchable.
In conclusion: Teatrino is great, and by itself is worth suffering season 1. Maybe some of you have noticed on this board that I'm a very harsh critic, and so I should say that among the very few things I'd give a 9/10, Teatrino is one.
oh and the last episode of Teatrino is very intense and emotionally engaging.