It's good. You get many many hours on lowish volume and at least a few on mid. It's rated for 6 hours, but you can get more out of it. (at first - as said, that doesn't last too long).
What's remarkable is that you can turn the battery off when the device is plugged in, to extend your battery life. Mine spends most of it's time plugged in on the desk. A simple flick of a switch and your battery is back on, you pick it up and take it with you.
UPDATE:After a year and a few months the battery isn't what it's used to be (it lasts about an hour or two on lowish volume). If portability will be your main use for this unit, as opposed to desk or kitchen use, you might want to consider further.