The Palm Portable Keyboard is actually just a rebranded version of the Stowaway Portable Keyboard from Think Outside (http://www.thinkoutside.com). Targus markets the keyboard (under the same Stowaway name) for many other PDA's. So if you have a Visor, Clie, Jornada, Cassiopeia or iPAQ, don't feel left out.
And it's great. It's basically a full-size touch-typeable keyboard that folds into something slightly larger than the PDA itself. Yes, it is as good as a laptop keyboard. You can even use the Stowaway/PDA combo instead of a laptop if you use the latter only for typing.