Yokohama Kaidashi Kikou is over, after 140 chapters. It's almost enough to make me drink a cup of coffee in Alpha's honor, although since I'd rather drink diesel fuel than coffee I think I'll have to pass. Maybe I'll have raise a beer to Alpha instead.
If you're not familiar with it, there's a Wikipedia article that gives a much more thorough summary of YKK than I could. But if I had to describe it in a paragraph, YKK follows the life of a human-looking robot, Alpha, and her friends, set at some indeterminate point in the future where the pace of life has significantly slowed. It's essentially a laid back, slice of life type series that pretty much throws conventional storytelling out the window, but it still manages to be compelling. If you liked Aria or Zettai Shonen, I'd recommend going to misago.org and cafealpha.org to check out the manga. There are four OVA episodes for YKK as well, which I liked a lot also. Having said that if you don't like loose ends and unresolved mysteries, and want a lot of action in your manga, then run away as fast as you can.
YKK is set in a future world where human-like androids, like the main character, Alpha, coexist seamlessly with humans, but humans are slowly dying out in a world that's much more sparsely populated than ours (and with a bad case of global warming). It's a world that's full of mysteries (just a few are listed on the Wikipedia page) but if you expect to find answers to any of them by the end of the manga, you'll be disappointed. Why is humanity slowly, but peacefully, disappearing? Who the hell is the Misago really? What's up with the trees that look like streetlights? All good questions, but you won't get any answers. The ending of YKK is more like an arbitrary point in Alpha's life where the story just stops following her--there's no big event, no climax, no conclusion that ties up all the loose ends.
With any other series an ending like that would probably solitict a lengthy, hateful rant, but when I think about it, it's an appropriate way to end YKK--much like Zettai Shonen the true nature of the mysteries it presents aren't important, it's the development of the characters that's the focus. And if YKK had ended like Planetarian, I'd never forgive the manga-ka. It is sad to see all the potential stories that never got to be--Director Alpha and what became of her, the backstory of the Misago, and so on. A determined fanfic writer could probably crank out a billion pages picking up where the manga left off. But as much as I wish we got more closure for all the characters in the end, an open-ended "ending" has its advantages. Would you want to see Sensei, Ojisan, Tadahiro, and all Alpha's other human friends die, as she remains ageless? It'd be interesting to see how Alpha coped, but chapter after chapter of funerals as time relentlessly marched on for everyone but Alpha would get depressing too. And not to spoil too much, but as a fan of happy endings I wasn't disappointed with the note the series ended on.
If you haven't checked YKK out I'd highly recommend it--like Zettai Shonen's Tana arc, it does a great job at creating a mysterious atmosphere, and has an interesting, endearing cast of characters. Most of YKK is scanlated at misago.org, with the last couple dozen chapters at cafealpha.org, and AnimeSuki has the OVA episodes fansubbed as well. It's one of those series it's hard to do justice to in a review, you really just have to see it for yourself.