Spoilers in this post.
Okay, I'm all for happy endings and stuff but I think some of you are taking this a bit too far. I mean out of all the comments I've read so far... it's prettimuch a general consensus that "the ending sucks!" or "what a disappointment, Yuu Watase!" or "don't read this because of the retarded ending!" I, however, disagree.
Just because it isn't a happy ending, or the ending that YOU want, that shouldn't discredit the fact that it was a good manga and that Yuu Watase is a good writer who really THINKS about her manga. It might not be the pretty ending that you want, but it's the ending the author wanted to depict and that should make you as readers think WHY.
Yes, Riiko and Night were in love, but Night was a figure - not human. Sure, Watase could have made it a sugary happy ending, but personally I feel like that would have made the manga less provocative. It would have been less believable, less moving, and less thought-compelling than her emotionally charged ending.
Think about it. Night is a figure - even Riiko truly realizes this in the end - which is why she has to move on. Night knows that he can't truly be human with Riiko forever either, because he will not age, cannot provide her with a family - cannot truly be her lover. He loves her but he can't provide her with a normal human love that he knows she deserves. Night severs all ties with Kronos Heaven so that he can be with Riiko, but he also knows that without their constant hardware updates and the like, he will die. Night knows that he can't give Riiko a normal human relationship because despite all his gallant efforts, he is still only a figure. So Night makes plans ahead of time to send a letter to Soushi when he dies, because he knows Riiko deserves a human love. Even if Night will one day wake up, Riiko cannot wait for him forever. Riiko says so herself that even if Night were to wake up someday she might be older or maybe even a grandma by then. It wouldn't work. Sometimes no matter how much people are in love, it just cant work out. Night was Riiko's first love but not her last love. Riiko's relationship with Night was a period in her life that made her finally understand love and grow as a person.
I think Yuu Watase's ending was the most reasonable conclusion to this manga. It doesn't spoil it's readers with "warm and pretty lies" that love is perfect - because it's not. Your first love may not be your last love. That's right people, love is not perfect and I think Watase did a beautiful job of depicting that with her bittersweet ending.