I think it's only not the same game with harder difficulty (and a bit of new content) if you didn't get the Hurricane Packs in the original. I will admit that it has some cool stuff to offer (more than just the old HPs), and Ninja Dog mode sounds cool, but I doubt I would go out and buy it. But anyway, I haven't played it yet, so maybe I'm wrong here.
EDIT: Well, from the reviews I've read, it is actually a lot different. Having the Lunar in normal mode without having to spend two minutes loading up the Hurricane Pack sounds nice. I still wouldn't pay more than $20-$25 for it, though.