Why have a central vac system? (What's wrong with my upright/cylinder cleaner?)
I saw them when I was in America once and always thought it was a good idea, plus I love gadgets!
There are lots of benefits too! Not least the fact that old air and dust aren't recirculated around the house. (these units vent externally, or into a non living area)
Additionally they are almost silent in operation, the unit needs very infrequent maintenance, the dust collector is so large that it only needs emptying every few months and you don't have to carry a large heavy vacuum around the house with you. Cleaning the stairs is especially easy (and especially difficult with a normal vacuum cleaner)
Step 1: Design
I spent absolutely ages planning this, I had originally intended to fit a central vacuum system when we first bought the house, but never did. Recently I've boarded out and reinsulated the loft, and it seemed like an ideal location for a central vacuum system, so the idea was reborn.
The main things to consider are the pipe run itself (and trying to hide as much of it as possible) the location of the outlets to provide best coverage of the house and the vacuum unit itself.