I really like the ending. It makes a lot of sense.
They've been saying since Season 4 that Dexter won't have a happy ending, and honestly I think this ending is one of the more upsetting endings they could have used.
Dropping Deb into the ocean makes sense if you think about it. It was Dexter ruining everything he enjoyed about killing, he will never forget watching his sister sink out of sight into the ocean. He can never enjoy the ocean again. That was the only thing he still enjoyed about his kills, and now it's gone.
Clyde Phillips' proposed ending wasn't that great. Sure, it was artsy and brought the story full circle, but I was speculating that ending when Season 2 was airing. Everyone says they want something else but if they had killed Dexter, people would be pissed. If they gave him a happy ending, it wouldn't feel right. Having Dexter survive and everyone knows what he is, would be a cop out
The only other ending I would have been okay with, but only slightly, would have been.. Dexter survives but loses everyone he cared about, and ends up in Astoria anyway. This one would have a really powerful emotional blow, and I'm relatively glad that they kept Harrison alive.