Like everyone says, they are a (little) bit cheaper than the high street, although I don't think that they are that much cheaper these days. About 5 years ago, they were THE place to buy stuff, now they tend to cater more for the mobile/DVD/"backup" software market, and there's less choice in the actual hardware available (IMHO).
Again IMO, they've been taken over by the kind of people that are ruining eBay. You can still get a decent deal, but you have to look around (and go to it a little later on...when the traders have clocked each others prices and started to lower theirs).
Shop around, but remember buyer beware...