Way overcrowded, too exhausting to navigate through, and almost intimidating in terms of the amount of merchandise. So much stuff (like shopping Europe and America), so little time. Every time I've been there in my 7 years in England, I've managed to get a headache. The Foodhalls are great though. (Do NOT, repeat, do NOT go on an empty stomach.) Just cruise the ground level and exit.
A better alternative would be Harvey Nichol's (Harvey Nicks), beloved of Ab Fab fans.