This sounds like a great new feature, but like everyone else said already, the annoyance of having to edit the ID3 tags in the song every time you want to change modes or play a new song kinda ruins it. Wouldn't it be better to just add the tags within Audiosurf, without changing the .mp3 file? And you can't even do this with .wav's and other tag-less formats...or does it use the filename in those cases?
There's another, more subtle problem: unless the order of the tags matters, you could create multiple setups that are the same, but won't have the same scoreboards because the tags are in different orders.
Sounds like a lot of fun, though...I can't wait to try 4 lanes.
