This review will take into account and be based partly on Audiosurf (the original).
TL:DR - If you like the first one, you'll like the second one.
The original AS came out and completely changed how I thought about Indie games. This was in the early, early days of Steam having Indie games on their store. AS may have even been the first; it is certainly the first I remember seeing. The polish was something I'd never seen on an Indie game. Everything (mostly) worked and it didn't look like garbage. More than I can say about most Indie games, even now.
As the successor, this had to be that, but more. I wouldn't be content with just the same thing, but more shiny. It is much more shiny, and with the inclusion of Workshop support, it is whatever you want it to be. Want it to look different? Someone may have already made your dream skin. If not, do it yourself.
The game modes are relatively the same and work by the same rules as the first one. If you like Ninja Mono, you are in a for a treat, as there are now multiple Ninja Mono variants. That is primarily what I play.
Mostly everything is much better. I say mostly because I do not like the way the spikes on Ninja now appear. They are hard-to-see circle/balls. But that is my only gripe. There are things that don't appear done, like the loading screen, but it is still in Early Access.
For being an early access game, it is remarkably solid and still has that tremendous sense of speed from the first one. It pushes my 7970Ghz to the max, but it is buttery smooth. Totally worth it. Doesn't seem to use much CPU, and still only 1 core, for whatever reason.