Personally I won't go anywhere near compatibles. Too many bad experiences. A couple of good printers I've had have been affected by them - not broken, as such, but the print quality has never been the same after using them.
If it's on the grounds of cost, there's plenty of websites which sell originals at much lower prices than your average high street store. Personally I think anyone who buys an ink cartridge from PC World is daft (unless it's urgent) - why go out and pay a ridiculously inflated cost when you can order one online for about half the price and have it delivered straight to your door the next day?