In reply to sarahjk:
The £40 quoted will include the cartridges and when you need to replace the pads they are about £10/pair. Many people consider that Swiss Stops provide the best braking in terms of stopping power and modulation while also minimising rim wear. There are obviously a number of other factors that influence braking performance including road surface, tyres, cables, rim condition and rider technique. A high quality set of pads are a relatively cheap way to improve braking performance.
I use the Sram / swiss stops which Merlin currently have on offer £28 for two pairs including cartridges
http://www.merlincycles.com/sram-by-swissstop-brake-pad-holder-1-pair-68078...
There are other decent pads to be had for less money and, like many things in cycling, the question about whether swiss stops are worth the money is a personal choice. If I'm approaching a corner on a fast descent and realise I've left the braking a bit late I would rather have top quality pads than lesser pads and bit of extra cash.