Only NEC has not upgraded from the standard mobile phone keyboard. The keys feel quite dead, although they are easy to use given their size and spacing. Of the dedicated phone keypads the NEC, Nokia, and Blackberry are quite usable -- the Sony, and particularly the Siemens, less so. Making a phone call on the other units involves either using their QWERTY keyboards or the touchscreen.
The Treo's QWERTY keyboard is fairly horrible -- as is the i-mate's. For making phone calls the buttons are simply too small and close together -- most users would find this quite frustrating. In both cases you are far better off using the touch screen to dial a call. This is also quite easy to do on the O2 and Motorola, phones without a physical keyboard.
Inputting data for PDA functionality is met with varying degrees of difficulty. Let's start off controversially by stating that the physical QWERTY keyboard for most of the phones are downright horrid.
Arguably the worst QWERTY is the Siemens, and it sadly has no handwriting recognition or onscreen keyboard alternative.
You have to commend Siemens for trying, you can see what they were trying to do, but the keyboard misses the mark.
The other phone that vies for the worst keyboard award is the Blackberry, but then maybe we are being a bit unfair because once you get used to the weird input system it is probably quite good.
The keyboards on the i-mate, Sony, and the Treo have tiny keys. With deliberate perseverance it is possible to become reasonably adept, but if you do not use the feature all the time it could be a pain. There is also an onscreen keyboard but it is not really any better.
With the NEC you only have the standard alphabetically mapped keyboard to input names and data. This is no doubt the slowest and most painful of all the modes.
You would only really want to use this mode in an emergency. The rest of the time you would be better off inputting data into your PC and synchronising it with the phone -- obviously the best option for all the phones.
Easily the best data input keyboard of the group is the Nokia. Its keyboard and keys are quite large and well spaced -- you cannot touch type, but when two-finger typing you can have confidence that the key you poke will be the one that registers.
The Motorola, O2, and i-mate also have handwriting recognition as input methods. The the O2 and i-mate are not too bad, but the Motorola is not that great.
Navigation
Navigation methods vary. The simplest operation is the scroll wheels used by Blackberry and Sony that allow the user to scroll through the screen icons and select the one they want. All other phones have at least a cursor pad and select key to navigate the options with.
The i-mate, Motorola, Sony, O2, and Treo all supply touch screen navigation via small styluses or your finger. Using this mode the Motorola is the clumsiest. The Sony and Treo are better but arguably no match for the more familiar Windows-based interface of the O2 and i-mate; particularly when you start to load the phone with goodies and have to navigate through them. The odd ball of the group is the Nokia -- it has a large display but it is not touch sensitive. Instead the Nokia relies on a series of navigation buttons, but the menu system is so well laid out the system works remarkably well.
Although it is not quite available yet, you must also check out the next HP iPAQ 6500 - it is the size of a Blackberry 7230, but so much more capable; quad-band phone with EDGE, 1.3Mpix camera & LED flash, Bluetooth (no WiFi), GPS!!!!, and the usual PPC2003SE stuff (hopefully upgradeable to WindowsMobile5.0 in Oct or so).
http://www.brighthand.com/article/iPAQ_hw6500_Coming_in_September?site=SmartPhone
http://davesipaq.com/news/ipaq_hw6500_comparison.html
And here is a link to some recent forum comment about the size (smaller than a normal iPAQ), battery life (better than a normal iPAQ, not as good as a Blackberry!) and built in keyboard (very similar to Blackberry).
http://davesipaq.com/forum/showthread.php?p=35358#post35358
It is already released in Germany & UK (in theory at least!) and I'm hoping to see it in Australia in Aug or Sept..