I prefer to go to Waitrose as I can have a day out in Cornwall then (but have to pay 75p for the priveledge of leaving)
As Edward said, they have the same range of the normal fiidstuffs, but have a wider range of higer end, higher quality products. Great fruit and veg and the meat in there is really good.
You could always try Ocado if you just want to see what their stuff is like as they are he online delivery wing of Waitrose.