i use the apple bluetooth keyboard for my MBP & ipad, works great & battery lasts foreverrrrrrr.....ive had it for months now, leave it on 24/7 & still havent had to replace the first batteries i put in it, def recommend.
The most obvious difference between the Apple Wireless Keyboard and those, like the Zagg, that are designed to be part of a combination iPad case/keyboard is the size of the keyboard itself. Almost all of the combo devices limit the width of the keyboard to the width of the iPad (in landscape mode.)
Many folks who are accustomed to a full-size keyboard find the smaller keyboards cramped and somewhat difficult to use. This is especially true if you must move back and forth in your work.
The AWK is a very good keyboard. I use one in conjunction with the Incase Origami Workstation and the combination works very well. Logitech makes a similar keyboard/iPad stand combination for less money. And the Microsoft 5000 bluetooth portable keyboard is a great value for the money.