Well in terms of goods on offer, prices and delivery, shopto are fantastic. I pre-ordered Killzone 2 there in February and got it 2 days before release date. They have a policy where if you don't receive a pre-order by the official release date, you get a couple of quid back.
On the flip side there's been concern over credit card detail theft. Not sure where this rumour/reality stems from. However, last month some £200 was withdrawn from my bank account via an online horse betting company. Now, putting 2 and 2 together, I still wouldn't connect the two, but it's the most likely scenario in my head at this minute. I've used nothing but secure sites and haven't flashed my card in public. Thankfully I've got my funds back now so all's well.