(Copied from the review I left on MyAnimeList)
The Big O starts out a stylish, oddball fusion of hardboiled detective fiction and tokusatsu style superheroics, but gradually evolves into something altogether weirder and more existential. It is partially successful in this.
First, the positives. The Big O aims to capture a very particular vibe, and succeeds. It's got interesting shot composition, a distinct visual style, and an achingly cool soundtrack of mostly jazz and funk. Most of the episodes consist of Roger looking into some case on behalf of a client, occasionally aided by an arsenal of gadgets, before a climax where he gets into the Big O and fights another giant robot in the middle of the city, and the show absolutely sells the idea that these two things belong together. The Big O itself is a fantastically designed super robot, and you get a real sense of it's weight and immensity whenever it's on screen. The Big O itself, and the rest of the setting, has a wonderful retro futurist analogue technology feel, all buttons and switches and levers. Over the course of the series the function of each control in Big O's cockpit is clearly shown, which makes it feel a lot more real than many other mecha. There's a lot of fun, sparky chemistry between the main cast of characters, particularly between Roger and the deadpan android Dorothy.
However, vibes only go so far. The stories of the standalone episodes often feel truncated to accommodate the mandatory giant robot fights, and the main plot is often not well explained, particularly the ending which may leave many viewers completely lost. There's also an excessive amount of time spent on characters having conversations which sound mysterious but tell the audience nothing, which adds to the vibes, but detracts from one's ability to understand anything that's happening, and twenty-six episodes is arguably too long for a show taking this kind of approach.
The things Big O is certainly worth a try for fans of mecha, or those who are intrigued by it's unique style, but if the first episode doesn't hook you in I personally wouldn't recommend continuing with it; everything that makes the show good is right there from the off.
6/10