At the end of the series back in 2009 I wrote about my uncertain feelings about its ending, but I was of course completely oblique about it all for fear of spoilers. The whole thing has stuck with me since then, and today it came bubbling out on Twitter so I'm going to dump it here as well. This time around there are spoilers and there will be even more spoilers in my elaboration after my tweets.

(Probably no one cares about DtB: Gemini spoilers by now.)

@cks_anime: I still like Darker than Black's 2nd season. How could you not like a confused, emotionally numb teenage girl with an anti-material rifle?

Also, I continue to not know if the ending of Darker than Black S2 is a good one or a terrifying one. (It's probably supposed to be good.)

@Evirus: Did the OVAs help?
[...]

Unless they touch on the nature of the alternate world, prob. not (for this). Maybe not even if they do; it's a philosophical issue.

Alternate/second world Suo has at least completely different memories than DtB-world Suo. Is she the same person in any real way?
If she isn't, then DtB-world Suo dissolved away to death and we have some fake clone thing wandering around in the second world.
It's kind of nice that there's some second-world Suo enjoying a nice life (along with versions of other characters), but ...
... it's not really a happy ending for many of the DtB-world characters like Suo. They're pretty much all dead or otherwise nothing.

Unusually, the Wikipedia page lacks its usual spoilers, so I must describe things from my memory. At the end of DtB: Gemini, we discover that the original Suo died in a terrorist bombing as a child and the Suo we know is an imperfect copy created by Shion and stabilized by the meteor fragment. With Shion reaching the end of his life and powers, Suo's life (or if you prefer 'life') is at best limited, and I think the meteor fragment was decaying as well. At the end of the show, Shion uses the last of his powers to help Suo as she dissolves away, and we see another Earth hanging in the sky visible from the weird place Shion and Suo are.

We cut to that second Earth, where a 'Suo' is living a perfectly ordinary happy family life with her parents, and I have a memory that we see versions of some other now-dead characters as well. It is clear that the Suo of this world does not remember anything from Gemini and has led a completely different life. If this actually is the real Suo from Gemini, transported to this second world in the sky by Shion's last actions, she is at least a completely different person in every practical sense. The Suo of Gemini has gotten a happy ending by losing everything that made her the Suo that we knew.

Is this really a happy ending? I don't know. I suspect that it was intended to be at least a somewhat happy one (after all, Suo lives). But it's not the Gemini Suo that lives; it's some stranger who has a tangential relationship to the real Suo beyond sharing her name and having parents with the same names and so on.

That's why to me it's at least partly a philosophical question. Suo's soul probably survived, but she wound up as more or less a different person. Is this a happy ending or basically a tragic one, because the real Suo is more than a soul and the person that was Suo is dissolved away now? I still don't know what my answer is here, but I can't feel that the ending is any better than bittersweet at best.

(Similar questions apply to other characters in the second world if we assume that Shion shifted their souls over to the second world along with Suo's.)

PS: After all this time it's possible that I've mangled bits of the ending of Gemini, as I'm relying on my memories here.