oh yeah and you'll normally waste about 4 litres per 1 litre o the good stuff
Mmm, wouldn't agree, at worst the waste to pure ratio is 3:1, at best it can be 1:1
With the system that a certain Roy Harding has, his waste to pure ratio is 1:1 and his R/O is bringing his TDS reading down to something in the area of 0.001ppm.
A very effective and efficient setup.
If you are on a meter it will increase your bills of course, but by how much will of course depend on how much water you use in a day.
The daily cost of this is very low however.
As Tosh said, Matt's DIY site is a great place to start, lots of very good info.
Another one with many FAQ's and information is:
http://www.window-tools.com/cgi-bin/yabb/YaBB.plIan