Echoing other responses here: writing on it is practically lag-free (there's a tiny amount of lag, but I don't even notice it anymore); using the eraser tool to

Echoing other responses here: writing on it is practically lag-free (there's a tiny amount of lag, but I don't even notice it anymore); using the eraser tool to erase sections takes a little more lag, using the move tool to move sections around lags a bit more, and a full page refresh can take a couple of seconds. In practice, it's been perfectly usable (for me). The only nit I've found responsiveness-wise so far is the startup time. It's not convenient when I suddenly remember something and want to write it down and have to hold down the power button for three seconds and then wait another five or more for it to get to the main screen and then wait another two or so to get to my scratch pad.