Works more or less the same way as the GW2 version (which I love).
As others have stated though latency will play a role in how well you perform (pun intended). That should go without saying.
The amount of buttons doesn't concern me, as you won't use but a portion of them at any given time for most songs you want to play. What does concern me though is the lack additional hotbars not added to accommodate this new feature. I know PC players in particular tend to use quite a few extra hotbars because of ease and KB/M making that efficient.
The lack of macros is negligible, as this type of feature is typically intended to be a manual thing anyways. You can just use a third party program if you wanted to automate it. I'd imagine SE would adopt the same view to it the way ANET did for theirs. I mean if they'll look the other way when it comes to parsers this should be fine as well, so long as that's the only thing you use it for. Just don't expect them to make an official statement one way or the other.
It would be nice if they either gave Perform its own set of hotbars or simply added more total.
Also, add some sound settings in the menu.