This anime is a must watch. First I will say that there is shounen-ai in this series. I do not consider myself a BL fan at all (nothing against slash pairings, but I just find yaoi to be too fetishized usually). However, the romance in No. 6 is done really, really well. I would say it's one of the best I've seen in any anime, and that includes all the shoujo ones that are specifically catered to be romantic.
In No. 6, the bond between Nezumi and Shion is essential to the show, but it does not overshadow everything else. The plot is fascinating by itself, and the supporting characters to the main two are very likable. Each one is fleshed out nicely and none of their backstories are cliché.
I've taken a look at both the novel and manga after finishing the anime. In my opinion, the anime is the best of the three because of how all the characters are designed and portrayed, as well as how the anime organizes and presents the story. People who like the novel and/or manga better may feel that the anime was rushed, especially towards the end. I agree that it is not perfect, by any means. However, the anime never feels boring. Every episode kept me riveted. Nearing the end of the series, I had wondered how they would manage to tie up a somewhat ambitious plot in 11 episodes, but upon watching the ending, I felt very satisfied.
No. 6 has become one of my favorite anime, maybe even *the* favorite. I'm a big fan of dystopian stories, as well as romance which is well-done and complementary to the plot-at-large. For anyone who is hesitant to watch No. 6 because they've heard that it is BL/yaoi, I will say that it's really not the main focus of the series. Plus, even though I'm a girl, I've enjoyed plenty of fiction which has shoujo-ai in it. IMO, if it's done well and respectfully, not in a fanservice-y way, then I'm all for it.
On a final note, I really love the opening for this show. The ending is good too. Please give No. 6 a chance! It's a hidden gem in anime, and I hope more people will discover it and enjoy it.