Large screen real estate has its advantages. It is easier to design presentation slides when you have a large workspace in front of you. Extra space also enables you to open multiple windows, for example a PDF file with comments on the previous version of your presentation, or an Excel file with the data that need to go into your pie chart.
Now, 27" is a lot of space (2550x1440 pixels) and for most ordinary people, one screen will do. A presentation designer might actually need two (putting her in the same category as financial traders, air traffic controllers and social media addicts). I like to design on a clean and calm canvas. All the small windows with bits of information distract me. So I use that second screen as my messy desktop, literally pushing bits, pieces, and windows aside when I do not need them, preserving my pristine and uncluttered design environment in front of me.
Now some technical details. An Apple Thunderbolt screen can only be connected to a recent MacBook laptop that actually has a Thunderbolt port. But more importantly, the dual screen configuration only works on the most recent 15" and 17" MacBook pros, not on the 13" MacBook Pro, and not on the MacBook Air. (This might actually be an argument for getting a MacBook Pro over a MacBook Air) at the time of writing, October 2011).
You need to install a software update for your MacBook Pro, and a firmware update for your screen. Do one screen at a time: connect a screen, run software update, disconnect, connect the other, run software update.
Once everything is updated, I found that the hardware works almost perfectly. You connect one screen (with a combined Thunderbolt and charging cable, the laptop gets charged via your screen) to the laptop, and connect the second screen to the first screen. Via system preferences you need juggle to assign which screen is right, which is left, and which one of the two is the main screen.
The Thunderbolt screen multiplexes in a number of USB ports, Firewire ports, and LAN connectors on the back of the screen to which you can connect other hardware (nice for a MacBook Air with limited connectivity). The screen also contains an HD camera and speakers, so the screen is not just a screen, it is a big hardware extension for your laptop.
I said almost perfectly. There are some glitches. Each time you connect the laptop after you return from a meeting, your Mac seems to forget the configuration (left, right, main screen). Also, when the computer is asleep for a long time, I find that the hardware connected on the screen ports (keyboard in my case) is no longer recognized, and my Internet connection is killed. The only solution I have discovered so far is to disconnect the screens, wait everything to return to normal (i.e., Internet), then reconnect, and then reconfigure left, right, and main screen. Hopefully this bug gets fixed soon.
But all in all, worth an upgrade.