Oh well. Spoilers for the last episode of the series ahead, naturally.
I didn't expect a happy ending, because experience told me that Dexter seasons never end exactly well. Most did end on a bittersweet note, with the notable exception of Rita's death in S4, which was a real downer ending.
So, the last episode certainly lived up to my expectations of leaving me a bit depressed. There is no happiness for Dexter. More to the point, there is no happiness for anyone. Debra's death was to be expected, given the way they handled her throughout the episode. But it was nevertheless very sad. And losing her, for real this time, sent Dexter over the despair horizon. And he didn't even succeeded in commiting suicide...
Deb's death and Dexter's assumed death leaves everyone else shattered, too. First and foremost Hannah and Harrison, presumably at least, and of course Angel, Quinn (his face said it all...), the rest of them.
It was almost comedic that Dexter committed cold-blooded murder, and I mean
visibly cold-blooded with really bad pretending of self-defense and panicking, in front of a police camera, and his colleagues believed him. Well, at least Saxon is dead, too. Too late for Deb.
But, apart from the overall impression of depression that was executed reasonably well, the last episode once again lacked in the details. Everything with Hannah, Harrison, or Deb's former boss left me absolutely cold. The flashbacks with Deb and Dex and the baby as well (mainly because I hate babies, even more so on TV). The callbacks to earlier seasons (ice truck!) could not salvage that.
Overall verdict: It's over. And please don't make a sequel movie or so - after all, with Dex, Hannah and Harrison still alive, that's possible.