The only options that use a real, albeit a slightly smaller, keyboard are the Clamcase and ZaggMate. All the others use a 'rubbery keyboard' that doesn't have a Shift key on the right side of the keyboard. The Kensington, Sena, and Brookstone all use the rubbery keyboard.
I am ignoring the huge one available at Best Buy, the Rocket something or other. Also there's the Crux360, but it's not available yet, pre-order only.
I've used all of these and returned all but the ZaggMate and Clamcase, with the Clamcase getting the nod for convenience over the ZaggMate. The rubbery ones are horrible to use and only marginally better than the virtual keyboard. Also note the cases and folios are only for the iPad1.
Frankly, I've started to carry the Apple BT Keyboard in my laptop bag along with my iPad2 (with a Smartcover and skin on the back). If I have a lot of typing to do, I will use the keyboard. If I need more than the iPad, I take my MBA11.
While it's obvious, when using any of these keyboards you can't tether using BT.