I'll add my support to everything that has already been said in favour of optical mice. However, although you theoretically don't need a mouse mat with an optical mouse, I find that I get better control over my mouse (for detailed work in Photoshop, etc) if I still use a mat. (Different desk surfaces can produce different results).
Also, unless PC World have suddenly dropped their extortionate prices, I can't agree with SRH that the products they sell are 'dead cheap'. I've used several makes of optical mice and the one I'm using now is probably the best of the lot. It's �5.47 in most branches of Tesco:
http://direct.tesco.com/q/R.100-1642.aspx
Chris