In short: the greatest game I've ever played.
The story mode actually had a decent storyline where you start out as a nobody in New Jersey to a skateboarding pro (yeah the whole start out as a nobody and become a legend seems common and overused in video games but tony hawk underground makes it work. The levels are much better than in the others too, with places like Manhattan, Vancouver, and Hawaii. I won't go into too much detail of the story or anything else (just find out for yourself if you don't own it) but it was the perfect difficulty (there are 3 difficulty levels though, I played on "normal" and it was the perfect difficulty).
I want to mention the level editor because...well...its awesome. It is so much better than the previous game's one (you can finally put in buildings, houses, roads, grass, tunnels, ladders, and much more). To top that off you can add in your own goals like "skate letters", "high score", "tricktris", etc. I made loads of cities on park editor and had just as much fun playing them as the real game (there is something I love about creating thing...all games should have level editors!).
I would write like 2-pages about it but I'm too lazy. So get out and buy this game if you haven't already. Did I mention that I noticed a graphical difference too?