The 27" is good for a dorm room, or a studio apartment where the owner will be using it as an all-around display for movies and gaming as well as computer chores.
For somebody who already has a big flat screen in their living room, I really don't see the point. It looks vaguely ridiculous sitting in an office. Like the owner is overcompensating for something. 😛 Actually it makes me feel the same way as when I see somebody (non-military) driving a hummer, or a Ford Excursion with no passengers.
Maybe it's from living for years in Asia and Europe. I dunno, there's a certain elegance to things that are well-proportioned and don't take up too much space or scream "look at me", but manage serve their purpose beautifully. Aside from movies, in what situations are users finding the 21.5" screen to be too small?
Before anybody chimes in with "I absolutely need two 30 inch ACDs side by side to do my digital editing work", let me just say fine, you need the real estate. But I don't think this is the case with the majority of users.