Much better then Daybreakers, first of all. It really comes close to true greatness in the first 2 acts, but then the end comes along and fucks everything up, literally. OK, so, I'll just start with a brief summary of the conclusion, then I'll get into to why it sucked. Adrian Brody fucks Dren. Why? Because she/it/later on, he, looks a bit like his girlfriend. Up until this point in the movie, he has been completely rational, ethical, person. Then, for about 5 minutes, he forgets all his logic and morals just long enough to move the plot forward. Then he's back to his moral, rational, self.
So after that piece of utter nonsense, Brody and...I want to say Ilsa decide to kill Dren, only to find her dead. OK, I accept that. They bury her, and then Brody's brother shows up with their angry boss. OK, I accept that. But then Dren comes back to life as a male. OK, I acce-wait, WHAT? OK, I know they technically explain this in the movie, but frankly, their explanation still doesn't make any sense to me. They say that Dren gender-bended because those organic blobs did it, and she was made the same way they were. My problem with that is the only reason the blobs did it and no one noticed until it was too late; was because the best scientists were paying all their attention to Dren! So if she started going through the same process, wouldn't they have noticed?
Anyway, the new Man-Dren basically spends the next 5 minutes killing the boss and the brother, and almost killing Brody. Then he rapes Ilsa. Just cause. I guess Man-Dren is just evil. I mean, sure Ilsa abused Dren, but why does that lead to rape? Why kill everyone else, and rape her? Why is Dren's solution to all problems to kill or fuck it? But I digress. Brody saves her not-so-just-in-time and together they finish Dren off. Uh, that is to say they kill him. The movie ends with of course now-pregnant Ilsa essentially selling the baby to science. Why? Because she's fucking insane. In fact, if, while watching this movie, should you ever find yourself asking "Why'd she do that?" then know that the answer is almost always "Because she's insane." Unfortunately, insanity is a pretty sloppy form of motivation from a writing stand-point, since it can be conviniently twisted to satisfy whatever the plot requires.
In fact, the whole ending really just consists of the same mistake repeated over and over again: People doing fucked up shit for no reason. Now, the "People doing fucked up shit" is fine, but if you're going to go that route, at least have the decency to put some motivation behind it. Put simply, Splice sure nailed the "fucked up shit", but botched(that is to say "ignored") the tangible motivation.