whilst no.6 is an enjoyable anime in itself, it misses out on a lot of the source material and as a result, becomes quite difficult to understand towards the second half of the series. after rewatching this show numerous times i had to turn to the manga and novel/light novel/whatever to actually get some answers as to what was going on.
despite this, i think no.6 can be enjoyed on its own as an anime, and has a great soundtrack, especially the ending song (although the opening is pretty good too). the characters are interesting and whilst key character arcs are missed, as an 11 episode series, it does an alright job at setting up who the main characters are.
would i recommend it over the manga? god no. but it's alright, it's enjoyable.