I've played hundreds of hours on Audiosurf 1 (non-steam), and past more than 100 on Audiosurf 2. The one thing to make clear is this: Audiosurf 2 is in beta and is not slightly, but extremely unfinished. I have updated my review to reflect my evaluation of the game as of the Dethrone update. In-depth review incoming:
===========Pros over Audiosurf 1=============
-- Better graphics, and skin mods.
The graphics possible in the new engine blow Audiosurf 1 out of the water without question. If you're an eye-candy lover this is great, and the integration of highly moddable skins makes the limit of eye-candy quite high.
It's without a doubt that the new engine being used is better and will offer far more if devs or mod makers take advantage.
Anyone can try their hand and creating their dream game mode, and this will surely be the lifeblood of the game after it releases. Whatever failings in design the developer may make in our eyes, the fix will be just a couple of code lines away.
--Separate score boards for modes
Quite a welcome addition. This puts an end to all the crying about "balance" (mainly from Mono players) of all the modes competing on the same score board.
--Support for many sound file formats like FLAC and some platforms like Soundcloud
The newest update to the game has added a friendly and easy competitive framework to the game. It's far cleaner and quicker than having to trudge through AS1's scoreboards. This in-game framework was not something available in AS1, and allows you to see what is popular per mode so you can be competitive with strangers even in unpopular modes. Definitely a welcome addition if you're competitive
That's it for the pros.
==========Cons below Audiosurf 1.===========
--Lack of the classic modes.
Right now we have Mono...lots and lots of Mono, the wakeboard modes, pusher, and a new version of double-vision. Yes, there is a Vegas fan mod (not very good), a classic eraser mod (doesn't feel right), some actually novel game modes (about 3), but for the most part we have LOTS of Mono, and a few mixes and mashes of said already official modes. Suffice to say, since the majority of the player base is mono players from 1 and 2 anyways, if you like Mono modes then this is your game.
--Lack of functioning key mapping for keyboard or gamepad
You would think that a functioning input scheme would be a high priority, but apparently not. Keymapping does not have any effect, and gamepads are only useful for mono since only the analog sticks and start button work in game.
--Unfinished UI functionality
The Dethrones update added a great boost to UI responsiveness which is now on par or better than AS1. The UI still needs quite a bit of work to be considered finished compared to Audiosurf 1. Game pads are not supported for UI navigation, only mouse and keyboard.
--Reloading tracks every time (even just for a track restart)
The capacity to save track schemes in Audiosurf 1 for quickly loading in your song is gone. Restarting your song before it ends also requires a reload of the entire track, very annoying.
That's it for the major cons I see.
Suffice to say, the biggest con of all is this:
The developer seems to work at a snail's pace averaging one update a month with the majority being skin functionalities inconsquential to gameplay or UI. It also needs to mentioned that the dev went silent for almost three months without a word about the state of the game's development. Community interaction is important, and while surprises are nice, it would be nicer if the dev took less than one minute in the future to at least say he's still alive and working on the game.
==========Should you buy it?=============
Overall, if you liked the variety and functionality of Audiosurf 1 I cannot recommend Audiosurf 2 for now. The Dethrone update was a leap in graphic quality and UI responsiveness and functionality, but variety the likes of Audiosurf 1 is still lacking in my opinion. Yes, there is wakeboard, yes there are some very new mode mods like Audiosphere, but in my opinion these are poor modes compared to AS1's. As some other reviewers have said, the game is lacking the puzzle game elements of Audiosurf one and focusing on simple mono like modes where powerups like paints have no place.
If you have never played Audiosurf 1 and are wondering whether you should get Audiosurf 2, I recommend you buy Audiosurf 1 instead.
If you don't mind the lack of variety, bugs, and general incompleteness of areas of the game that you have no interest in then buy at your own discretion.