I take it you are talking about boosting water pressure to your shower, right ?
If yes, a shower pump can make a tremendous difference to the water pressure you will get at the shower head, but it is not suitable for all types of boiler / hot water systems.
If you have what is called a combi boiler (usually found in smaller houses / flats), your shower experience is determined by the mains water pressure coming into the premises. Combi boilers don't have water cylinders or water tanks. They heat up the water the moment there is a demand for it. You cannot use a shower pump in this situation (it upsets the water companies )
If you have a traditional boiler (cold water tank in the loft, hot water cylinder in the airing cupboard etc.) than you CAN use a shower pump.
I'm no plumber, so for more professional advice you might like to visit DIYnot.com their forums are busy and you will find competent plumbers to offer more specific advice.
Oh yes, Put it this way, in our last house in North Yorkshire we had one. Bought this one and it didn't, before we moved in I did two things, installed a new boiler at the vendors expense and installed a power shower. A fiend of ours who travels the world, still likes coming to our house for dinner. He always says the dinner is great and the shower is better.