Fantastic, except...
Like so many others have said, lag is what kills the playability. I tried all your suggestions for reducing the lag, even downloading the Offline version and reducing the graphics/settings all the way down, it was still lagging.
While I had no problem getting used to the 6 key controls, trying to do a song on Normal mode is near impossible with any bit of lag. I've gotten a perfect several times in Easy mode, but I know I'd be able to get it every single time if there were no lag. I don't even want to think what Insane mode would be like.
Despite that, it's an awesome game.