Though quite cheaply made, this wool shaver has worked excellently for me for a few years, and I would buy two of these rather than one more expensive one so as to have either a spare or one to keep in a different room or the car. It needs to be used with a little more care than some negative reviewers may have done: use with a rotary movement and don't press too hard (to start with at least), and don't try to cut loose threads as it will pull them in.
I have renovated several pullovers with it, and it gives a very satisfying scrunch when it eats a bit of fluff. If it slows down before the batteries run out, it may have threads trapped in the centre of the cutter - just unscrew the cover and pull them off. Any gadget with a motor uses batteries quite fast, so I would recommend rechargeables.