Worst experience I ever had at the Apple store was the only time I've had the chance to go in.
I was on vacation to Chicago this summer and was staying at the Delaware Street DoubleTree, just down Michigan Avenue from the Apple Store, so I took a walk one afternoon to go pick up an iPod case...and of course to ogle the G5s and PowerBooks. I got my black iSkin eVo, but when I checked out I got a rather sarcastic employee who asked me if I "wanted an iPod with that." No doubt intended in good fun, but is that really the way to speak to customers? I told him I already had one and that it was working quite well.
Apart from almost hitting the glass stairs on the way out, that was the only bad part of the experience -- especially since I knew that the overly jocular clerk wasn't on commission. When I go into a place of commerce to do business I expect to be treated with respect -- especially at the very moment when I am forking over thirty bucks.
But then again, there probably have been people who have tried to buy the case thinking that the cardboard iPod was the genuine article, so the guy was probably not being as rude as he came across.