I've gotten all the way to the very last fight with Ares on Spartan Mode in the first game.
Of course, because the difficulty is so insane by that point, IO have yet to beat him on that setting.
BTW, the ending of the first game does not dissapoint.
(Edit) Okay, I'm going to add more now because I feel like typing. In my opinion, this game is one of the best examples of storytelling I've seen in a video game since the end of the Legacy of Kain series. Also, it feels like you are in an ancient greek epic poem, like the story could have been a part of the real mythology.
Also, the chain blades. In any other title, your weapon would be simply that; your weapon. But the developers took it several steps further. They're not just his weapons, they are physical ties to his master Ares. While having weapon the can be taken from you seems like a great idea, it is a constant reminder to Kratos of his dept and slavery to Ares, in the same manner that the ashes of his family, permanently bonded to his flesh, are a reminder of what was taken from him, what his quest for power truly cost him. It takes a character that is just an absolute monster, a person that you can't help but despise for what he is, and then show him as this deeply wounded character, who hates himself just as much as anybody else.
Whew. :)