Nice to see a topic about Planetes. Loved that series! The characters and storyline were top-notch. Loved the way these characters interacted, felt really natural and thus made for a very compelling watch.
Ai and Hachirota are definitely one of my favorite anime-couples. And that makes the ending happy and sad at the same time.
One small point of critisism though, is that I think it's a pity that Fee and Yuri were pushed into the background in the latter half of the series. Understandable, since Hachi was the focus, but I would have liked some more developement for Fee (since Yuri at least got some around the halfway point), she was one my favorite characters!
Also, I really disagree with the review of the series that's currently on this site (Arcanum reviewed it and gave it a 6 overall). I plan to write a review of my own sometime in the near future and give my own view on the series.