Picked this up last night and gave it a watch. Big spoilers for those who don't want to know:
Overall, a much more confident movie than the 99 theatrical cut, but I did kinda miss the bleached-out/mono chromatic look the theatrical cut had. Gave the movie a lot of style. Also, I was surprised to see that some of the "cooler" moments in the theatrical cut were created by Gibson (like Porter blowing up the car full of goons with his cigarette).
Maria Bello has a much more minor role in this one, she's more of a noir-era dame than a hooker with a heart of gold. The subplot with Liu is also greatly reduced, and she doesn't get her final smack in the face from Porter this time.
I must say, though, that Kristofferson's scenes in the theatrical cut made for a more interesting villain than Angie Dickinson sounding like a hysterical, Jewish Kathleen Turner over a couple of loudspeakers. I get not wanting to show Bowsen, but for Christ's sake, her line readings are awful.
The biggest changes come in the final act of the movie. There is no kidnapping subplot, and the climax happens almost out of nowhere (which, from the way it's presented, is the point). And there's one bit towards the end that made the movie for me--the shot of Porter slumped against a lamppost, hearing all the voices flash back through his head, bookending the beginning of the movie. Great scene.
I thought Payback '99 was a solid, gritty little revenge movie, and it still is. But Payback: Straight Up is a more rounded effort, and movie with some real bite, and one more reason why I wish Helgeland would direct more often (and when he does, it's not on crap like The Order.)