A lot of the "Director's Cuts" I've seen don't do much to justify their existence. They have a few "cut" scenes dropped in, but most of the time they interrupt the pacing and offer no real change to the film. The term has become synonymous with "Unrated Cut" and comes off as a marketing gimmick to get people to shell out for the same movie again.
I haven't seen the original cut of Payback since probably 2000, but I remember enjoying it. It was a gritty crime film with a good dose of humor and Mel's trademark "I love being tortured" shtick. I rented this director's cut to see how it played, since it was apparently a dramatic difference rather than a few new scenes thrown in here and there.
I'd say the new cut is a much better movie than the old one. It eliminates Mel's narration, which I think greatly improves the way Mel's character is explored. We get to watch it rather than hear it, so he is now Porter rather than Mel Gibson. One of the strengths of this character is that we don't fully understand his motivations (see also Point Blank with Lee Marvin); we have the duration of the film to try and understand his code, his principles, and the original narration spoiled the process.
Another change is the filtering. The movie is no longer entirely washed out blue, it's full of warm tones as well as cool. I like this as well. It still fits the mood of the movie, and it's more visually pleasing.
As for new scenes, I couldn't really tell what was added in. I could tell what was removed, which was the kidnapping sub-plot, Kris Kristofferson's character, and the subsequent torture and escape. The new cut features a tidier finale, and the whole movie is actually shorter than the original cut. It clocks in at around 90 minutes, which works for this story. I remember liking Porter's escape scene in the original (clawing his way through the backseat), but I think the new cut keeps the pace consistently and serves the story better.
It's definitely worth checking out.