UK stores are rather nice in my experience. Certainly the Regent's Street is fantastic - if a little overcrowded with buggers just using it as a free internet cafe. The new Kingston store is very nice also. Very open plan, more reps than customers usually. Fantastic.
Genius Bar appointments are invariably booked up every time I've tried to get one, but such is life. Apple are looking at expanding apple shop numbers n key cities - there was also a move 6 months or so ago to put them in larger supermarkets (Tesco Milton Keynes being the flagship) with a few reps, demo machines, but essentially running a front to the online store when it came to actual purchasing. Not being anywhere near Milton Keynes though I have no idea if it's still looking viable as a concept. Other than that quality of the Apple resellers vary massively, with some being Apple Store's in all but name, and others (PC world springs to mind) being utterly crap having a macbook on a shelf somewhere at teh back of the store, badly lit with grubby touchpads.
I'm moving to New York in the morning from London so I shall have to see the difference between 5th Avenue and Regent's Street in the flesh!