With PC World, it all depends. If you know what you're doing and the price is good then they can be OK. I bought a Netbook from them a few months ago because they were the equal cheapest, including on-line. After a couple of weeks, I tried using headphones with it and discovered that one of the channels wasn't working. I took it back to PC World and they replaced it on the spot without any quibbles because it was inside their 28 day return period.
Some of their brown box (as opposed to retail-branded) hard disks and DVD RW players can be good value.
It's just daft to say don't buy from PC World. Like anything, do your research first before you go there rather than rely on their advice. If their price is OK then way up the pros and cons of buying (and potentially returning) goods from a shop versus on-line.