Hmm - 'power shower' in my experience means something different to what Jimbo says - perhaps its regional (I am a native Brit from the south east).
The box on the wall in the shower enclosure, which heats the water itself, is what I know as an 'electric shower' and as mentioned above generally does little more than dribble warm water on you. They aren't great, but are used where the water heating system cannot support a proper power shower (i.e. some of those are not appropriate for house with combi boilers for example), and/or because they are relatively cheap.
A power shower, to me, is where there is a separate pump to increase the water pressure in a cupboard, which then delivers a stronger spray in the bathroom. I have one of these myself, and as you say, the motor is quite noisy. I am probably jinxing myself by saying this, but I've had mine installed for over 6 years and have had no trouble at all with it.