I don't tend to buy whole builds from OCUk, as previously mentioned being a couple of quid dearer across a whole build soon adds up.
But for individual items, and especially the possibility of something I want popping up on Today/Weekend/or This Week Only deals, in the next week or so, I tend to regularly check OCUk and order the odd few bits and pieces.
Christmas offers were especially good, picked up 16Gb RAM for £39.99 shipped, when everywhere else wanted £60+.
As consumers, almost all of us shop where we think we get the best deal, and in OCUk's as stated by most people, is that the best deal is not necessarily the cheapest, but the one that includes great customer service.