I am stunned by some of the responses here. Walk-ins should be welcome as well. NEVER turn a customer away. If you don't have an appointment, then you are in the "as available" line. I have a bunch of Apple stuff and would never think of meeding an appointment. Having appointments may be great for the Apple people, but it is very rude to customers. We don't all have scheduled lives, and things don't always break on a scheduled basis either.
Bad customer service is just that- bad customer service. And turning a customer away when they need assistance is bad customer service.
I intentionally keep the literary requirements of my posts to a level suitable for those of roughly 13 and above, yet I still receive responses like this one. I suggested that in such an appointment system, all you really need is for staff to guide the customers, either making an immediate appointment for them, or indicating when they'll be able to speak to someone at the genius bar. For minor issues, one of the floor staff should be able to handle such complaints. If someone wants to take off the back of their computer, what could possibly be wrong with the employee pulling out their issued iphone and making such an appointment on the spot. If the genius bar is not booked up, there's no reason this person wouldn't be able to see them immediately. It's just a matter of organization, and it helps ensure that those booking online prior to their visit enter the store with a realistic idea of how long it will take to speak to someone. I've visited many Apple Stores, and so far this seems like a massive improvement over their prior system.