Let me start off by saying that God of War III is an impressive feat. The scale is huge, the music is epic, the graphics are amazing (with a few oddball exceptions), Kratos is as amoral (or rather as immoral) as ever, and the action is fast and furious.
God of War III doesn't change the series' winning formula so much as it tweaks it to make it an even more solid experience. Now the jump and roll commands are actually valuable. In fact, jumping and rolling were the two things that saved me in many a boss fight. I also liked the ability to utilize enemies as weapons. This added another element of strategy that kept things fresh and interesting.
If there is any weak spot in God of War III, it might belong to the story. It's pretty clear that the developers were trying to tie loose ends as fast as they could and ran out of time before they could really smooth out the details. That's not to say it's terrible. In fact some parts of the story are fantastic. It just feels a little rushed. I'm also scratching my head over the inconsistency in graphics. The character designs for this game are all over the place. Kratos looks amazing, as do Hephaestus and Hercules, but for some reason the character designs for Helios and Hermes are really lacking. The same thing goes for environments: some levels are breathtaking in scale and beauty, while others are bland and elementary looking. It's nothing too awful, but it is a bit jarring.
At any rate, despite a few issues, God of War III is a solid effort. If you had fun playing the other three GOW games, (and love Greek mythology as much as I do) there's no doubt you'll enjoy the final chapter in Kratos' story.