We tend to think of space as pristine and empty, but increasingly, debris and junk orbiting Earth will be a growing concern for space-goers in the future. Planetes is about a team of people who handle exactly that problem. Even though their work is important and saves lives, they're kind of in the background, so they're not respected as much as "real" astronauts. I thought this was interesting, because there are a lot of jobs like that now, in real life. I mean, people don't think about or respect sanitation workers, but most people would miss them if they weren't doing their jobs anymore.
The main focus is on two characters. The first is Ai Tanabe, a newcomer to the cleaning crew, or Debris Section. She's basically like Leela from Futurama, being the one who pushes the others to be more serious. Then there's Hachimaki, whose name is a nickname meaning 'headband', because he wears one during his missions. Hachimaki is basically a typical 'red oni' type, who dreams of owning his own spaceship, but is held back from this dream by his job not paying well. While other characters make up the Debris Section, Ai and Hachimaki are usually the focus of the action and suspense.
Manga by Makoto Yukimura
Planetes is both very funny and very intriguing, and the characters manage to work well in both funny and dramatic situations. It's also cool visually and in term of space realism. Not many science fiction anime depict space travel realistically, simply because realistic space travel is not exactly glamorous (adult diapers are rarely brought up, for example). So, if you're like me, you might find it refreshing to see an anime that does actually take the time to be scientifically accurate and realistic.
As for what I didn't like, the characters can be a bit bland at times. None of their individual personalities stuck out to me that much. It's not like Cowboy Bebop, where every character has a stage presence by themselves. The cast was much more "average Joe/Jane" than that, which can either be a good thing or a bad thing, depending on what you're looking for in an anime.