The cheap high-wattage voltage converters should only be used with hair dryers or curling irons -- essentially pure heating appliances only. I'm not 100% sure, but I think they work by using a diode to convert the 240v AC to a half-wave output. This will really foul up any electronic device, and lots of items (like electric toothbrushes or shavers) have electronics in them now.

Virtually all electronic items have multi-voltage adapters, so you just need the plug adapter.

If you need to power something that's electronic but doesn't a universal adapter (microwave oven, for example) you'll need the expensive and heavy transformer.

If you just need a hairdryer, you're better off just getting a dual-voltage hairdryer.