Update: My Ultranav just arrived. First impressions:
Pros:
-Awesome!
Its uber-convenient, if you're looking for a portable keyboard.
-much lighter than I expected, based on the pics. It weighs almost nothing. Very comfy to toss in backpack or put in lap. Feather-weight.
-Much thinner than I expected based on the pics. Again super-easy to throw into backpack.
-nice and small - roughly the same as a sheet of 8x11.5 paper. And if you're using it as a keyboard or trackpad replacement for your laptop, you can place it directly over your laptop keyboard. Fits nicely.
-yet, its sturdy. Doesnt feel like its going to fall apart or flex much.
-love the trackpoint plus trackpad plus 5 mouse buttons. Very convenient. The trackpoint is classic IBM - very responsive and intuitive. I didnt realize how much I missed trackpoints (and how much the endurapro trackpoint blows in comparison) until I just used this and its like oh yeah, wow.
-trackpad is wonderful too, very responsive. They're both so good i cant decide which to use.
-Love the built in wrist rest, very convenient especially when its in your lap.
-has "2-level" legs on the back if you like that kind of thing.
-has built in volume and mute keys for what its worth. They dont work out of the box tho, wonder if IBM has some software/driver for this. They probably do, will have to look for it on the ibm website.
-definitely looks v. cool.
-cord is loooong. This may be a con actually.
-a standard ps2/usb adapter from radioshack seems to be working fine. You need one of those Y adapters since it has 2 ps2.
Key feel:
-not bad really. I know its not the "classic" ibm thinkpad feel, and I know the M4-1 is supposed to have that classic feel, (and I agree that classic thinkpad feel truly is pretty great for a laptop keyboard). However, its not bad, its not so mushy or anything that you dont want to type on it. Its fine for a portable. Like someone mentioned, I guess its like the newer lenovos, the R60s and so forth. Its not bad. Throw is deeper than you expect for a laptop board, and tactile feel is definitely there and somewhat crisp for a membrane based laptop board. Definitely on the better side of the spectrum for such a convenient board.
"Cons: "
-when numlock is enabled for your full sized board, then you plug this guy in, numlock is enabled here too, so you'll have to always disable numlock (alt-shift-scrolllock). This isnt a 'problem' per se, but if you have a regular size board at your desk, chances are you like leaving numlock turned on by default.