More or less what Ady said.
I've mostly used OCUK, only problem I ever had was when one item wasn't in stock and they didn't deliver the whole lot and didn't tell me about non-delivery. That meant I stayed in all day for nothing

Other than that, they've been good, quick RMA's and refunds for faulty goods.
However, I've noted other people's problems and I'm wary.
Lately have been using Scan and E-Buyer. I never thought I'd recommend E-Buyer but they seem to have turned around completely from the crap E-Tailer they were, let's hope it lasts.
And of course, as ever, if you want real service and don't mind paying a little more - Kustom PC's and Chillblast - by far the best online computer parts suppliers
ever.
In my experience, of course