* how does the display compare (brightness)?
When I bought my SLC760 the Zaurus had the only full VGA display on a PDA at the time. That, the keyboard and Linux were my main selling points, and I was (and still am) blown away by the device.
Now the screen on the 770 is another step up. It is *very* bright but also *incredibly* sharp and high-definition. It is physically larger than that on the Zaurus but actually "feels" even larger than it actually is.
* is the screen resolution of 400x800px a recognizable improvement?
For Web browsing in particular (its main function) the difference is unbelievable until you see it. I did not expect it to be that drastic beforehand, but those extra 160 pixels of width are incredibly effective. On the Z you definitely feel you are browsing on a PDA (albeit a very good one), and many sites are simply not usable unless they have a specific "mobile" version. On the 770 it feels more like browsing on an old PC or laptop with a small screen, and it's easy to forget it's actually such a small and limited device.
* how good can it be used without a built-in (clamshell) keyboard?
As a former Psion user, and still using my Z, I am biased keyboard-wise of course, and I miss it on the 770. I don't much like the virtual keyboard (although it's one of the most usable I've seen) and haven't even tried the HWR. As I use the 770 mostly for browsing and reading, data entry is reduced to the absolute minimum and it is not too much of a problem. But if I were to switch over completely I would definitely add an external keyboard (BT or USB through powered hub) when needed (just imagine using the CLI in an xterm without it... :-)
* what about operation time?
You mean battery life ? As usual that depends on what you do. Reported autonomy is 3 hours with continuous use of BT or Wifi (I haven't verified this, I tend to use it for shorter periods). There is a nifty button that switches off BT and Wifi when you're not connected to conserve power. Screen usage is probably determinant too (some have reported that playing videos or mp3 with display on eats battery quite fast). Reading e-books (another popular use) is probably easier on the machine.
All in all, I'd say that with a typical mixed usage (connected/unconnected, on-and-off) the device should easily last most users through a regular business day. I've certainly gone much longer between charges. A long train or plane trip, or intensive surfing sessions, are probably something else though.
The small cell-phone-type battery certainly doesn't match the capacity of the bigger one on my 760 ; then again it's probably easier and cheaper to carry a second one if needed.
* device handling (thicker, thinner, larger, smaller)?
The form factor is very nice and natural to handle. It is wider but thinner than the Z and fits the hand well. All buttons are on one side and can be pressed with thumb and index of the hand holding the device, with the stylus in the other hand.
* how stable and complete is the software compared to a Zaurus Sharp/Qtopia ROM?
I use Cacko 1.23, not Sharp, but still this is definitely the weak point of the 770 right now. I consider Maemo (the Gnome/Gtk platform) as beta quality at best, with a lot of work to be done, which is not so surprising.
The version of Opera that comes with the 770 is more recent than Sharp's and is definitely better at handling a majority of sites. But it also has the same habit of crashing and disappearing suddenly altogether, only worse
The Mail app (which I don't use, Gmail works fine) is said to be very buggy too.
The rest of the software (RSS reader, photo, mp3 and video players and a few games) seem to be OK. Chess players say the chess game is quite good.
This is not supposed to be a PDA though, so there is nothing comparable to Ko/Pi and no synch with desktop apps. I still use my Z for this.
On the bright side, the important thing to me is that the platform seems very open, much more so anyway than Sharp's. Nokia sound keen on involving the community, and ported software (text-mode or from Gnome apps) is coming in at a good rate, probably faster than when the Zs came out if you consider the number of devices actually in the hands of developers. Games (Doom, ScummmVM, Tetris etc.) look incredibly good on this screen ; xterm, mc, ssh, vnc and such are available. It should turn out to be a pretty lively platform if hackers enhance it with Nokia's help and benediction instead of swimming upstream like Z hackers did with Sharp.
* how is performance compared to a Zaurus?
If you use it as an Internet Tablet as it's called, ie for web browsing and a few other things, it's actually quite good, given the small CPU and memory size. Abuse it just a little though, and you really hit that 64MB wall.
On my Z I have a complete Python distribution with a web app framework running as a daemon that I use daily with Opera. I certainly couldn't do this with the 770 as it is now (even if it had Python, which it doesn't - yet).
In a way, the first 770 is like the first clamshell Zaurus, the 700 : a great and innovative device, but still a prototype with limitations you had to live with. The 750, then the 760, improved on that. I hope Nokia similarly persists and gives us the 780, 790 etc. with the same core qualities but less limited by hardware and thus more versatile.