I never had a terrible urge to buy any of the original 4, but going on what I saw in a demo disc trailer for Underground, this one should be the best of the lot.
Just one thing that annoyed me: On the trailer, video of someone holding their board between their legs and shimmying along a wire from one skyscraper roof to another.
I'd quite like to see a downhile obstacle course race in a Hawk game. I can imagine it being alot more fun than Graffiti mode, anyway.