The Big O is a very eclectic experience. It's as moody as it is humorous. It's as atmospheric as it is fast paced. And it's as lighthearted as it is existential. It follows a formula similar to a saturday morning cartoon, where the hero is usually pressed in to a corner, and at the last minute pulls out the big guns and saves the day. But it works so well here because there's so much more substance elsewhere in the show.
It overtly pulls inspiration from film noir tropes, making this perfect for anyone who enjoys the gloomy jazz and blues vibe that Cowboy Bebop executed so well. It does however, take a different direction with the second half. I don't say this to ruin it for you. I only say it to prevent any disappointment. If the first half can be related to Bebop, the second can be related to Evangelion, with all its existential questions presented through really trippy "wtf am I even watching" sequences. However, the lighthearted nature is never truly lost.
And of course, there is the Mecha element. At first it's more or less just treated as a plot device, and not really the main focal point of the story - like how the Tardaris is treated in Doctor Who. As the show progresses in the second half, it's brought more and more in to center stage. The Big O is great even for people who may not like the mecha anime, and could even act as a good primer for future interest in the genre.
It suffers superficial damage from age, but I still adore this show. There's so many things to pick apart and discuss. But the best thing I can recommend is that if you watch the first episode and just aren't feeling it afterwards, then this might not be the show for you. But if it is the show for you, you'll know very quickly.
Also, Steve Blum as the MC if you watch the English dub. Can't go wrong there~
