What are the best physical keyboards for the iPad?

The keys are great to type on and offer comparable press depth to the standard apple keyboards. From [Trusted Reviews][1]:

Layout and key spacing is superb, with number keys doubling as iOS function keys, a full-size right-shift key, and the welcome rarity of a full-size UK Enter key. Everything is exactly where you would expect it to be, and the shallow ridge that runs around the outside disappears to leave your thumbs unhindered access to the space bar. The space bar and other keys on its row are also vertically elongated, making them much easier to hit.

Feedback is equally impressive, with lots of travel and nice firm action to each key. What really sets Logitech’s Ultrathin Keyboard Cover apart from the majority of iPad keyboards though is that the keys are larger. Where the average keyboard case has keys that are 12-13mm across, those on the Ultrathin are 14mm, which makes all the difference in the world.
[1]: http://www.trustedreviews.com/logitech-ultrathin-keyboard-cover-for-ipad_Tablet_review