I received my Zaggmate/Logitech heyboard case a few days ago and find it SO useful. When typing on the iPad keyboard, I had found myself repeatedly making the same errors, eg. hitting the "n" key instead of the space bar. For forums, email, Documents etc, this external keyboard is a must for me.
Besides the iPad specific keys mentioned above, I just love the directional keys - the "bubble-dragging" to position the cursor drives me nuts on the on-screen keyboard. The "cut, copy & paste" dedicated keys operate in one step, without the need for pressing any "function" key at the same time. All of this makes the whole typing experience far more efficient for me.
For daily use (running around) my iPad is covered by a leather Yoobao case, when I won't be in need of the keyboard. These two cases together are the perfect combination.