In summary: very smooth, much better than 10.1 to 10.3 because I didn't try to do it on line, but with an upgrade DVD.
Minor problems with the graphics card driver and monitor but they were easy to fix.
Some niggles with things like DNS but again, it was just a case of redoing tweaks from 10.3 that had been undone by the upgrade.
Before you start
Try the Live CD.
I used this to verify that Novel-specific stuff like cl4others and mount.ncp still worked, and that key things like the network and display worked.
It was also useful to test why my monitor resolution dropped (more of that later).
The Live CD is 64 bit or 32 bit only (i.e. there are two types) but the install DVD is 32 and 64 bit in one.
The live CD is also KDE4, and a very early version (4.1 I think). Having experienced the 4.x series at home, I wouldn't really want to use KDE4 at work till 4.3 (and that's brand new), so I knew to make sure KDE3 was installed during the upgrade (the installer has GNOME, KDE3 and KDE4 on it).
See http://news.opensuse.org/2009/08/06/unofficial-opensuse-kde-43-rpms-and-live-cds/ however, for updated live CDs and post-install packages.
Take a note of your partitions (i.e. /dev/sda4 -> /home , /dev/sda1 -> /) just in case the worst happens.
Needless to say, take backups of /home/youruser, /etc etc.
Make a note of what X display driver is being used in /etc/X11/xorg.conf (more later) from experience this is the thing that normally goes horribly wrong.
Install
Booting the install DVD was straight forward, and it correctly located my install and gave me an upgrade option.
I had to resolve a few dependencies by hand, and the install also deselected all of KDE. I selected the KDE3 packages, changed from gdm to kdm and a few other bits and pieces.
The installer moaned a few times about '/dev/sda1' i.e. my hard disk being not a fixed name or some other such nonsense, but continued without incident.
Once the actual install started it took less than an hour to run.
Post install
On rebooting, I had not X :-(
I added 'x11failsafe' at the boot loader prompt, and this let X load. I suspected this was because X was using the ATI driver which was no longer installed, rather than doing something sensible like changing to the RadeonHD one that's on the DVD.
Anyway, either use YaST to select the 'RadeonHD' open source driver, or install ATI's via the one click install on http://en.opensuse.org/ATI#1-click_install
If you use ATI's note the final step of running SaX2, and make sure to install both the normal and -debug modules.
My monitor resolution dropped from 1600x... to 1200x... This happened when I first added the monitor to 10.3 too, so I knew to add some lines to the 'Modes' section in /etc/X11/xorg.conf from the backup I had of the old version of the file.
Via. http://en.opensuse.org/Additional_YaST_Package_Repositories I installed packman's MP3 and video codecs, as well as 'xine' so that Amarok can cross fade music.
Via http://download.opensuse.org/repositories/Subversion/openSUSE_11.1/ I upgraded Subversion to match Eclipse's version (v1.6)
The MDNS work arounds for Microsoft's cock-up over host names that end in '.local' had been reverted, so I had to add
multi off
mdns off
to /etc/host.conf, remove the "mdns.min... [NOTFOUND...]" section from nsswitch.conf, and then disable avahi-daemon (by running "chkconfig --set avahi-daemon off").
I stoped the avahi-daemon using the init.d script to save a reboot.
The pointless CPU hog Beagle was back, so I used YaST to remove that and it's FireFox plug-in.
I noticed then that FireFox was the 64bit version (and only v3.0 I think), so I added the build service's "http://download.opensuse.org/repositories/mozilla/openSUSE_11.1/" FireFox update repository and updated to the 32bit FireFox 3.5.
Apache failed to start, but this turned out to be because our custom Perl module (AuthAny) needed to be moved ("perl -V" lists new places to put it).
Font sizes had all gone a bit weird (over large) and I had to use both the KDE control centre and /usr/bin/gnome-appearance-properties to fix all my programs (Eclipse and Skype seem to use the latter. KDE's 'use my fonts in GTK' setting was being ignored).
Font sizes were still a bit odd (even 8 point was huge), so I poked at Xorg.conf and found I needed to get a ruler out and check my size, then correct the file:
...
Section "Monitor"
Option "CalcAlgorithm" "XServerPool"
#DisplaySize 337 270
DisplaySize 410 305
...
Compiz
All was good at this point, but I'd lost the fun of the Compiz effects. Getting this working was still a bit of a chore, but it's optional of course. At least KDE4 removes this fight !
I think what worked was adding both the repositories http://download.opensuse.org/repositories/X11:/Compiz/openSUSE_11.1/ and http://download.opensuse.org/repositories/X11:/XGL/openSUSE_11.1/, installing 'compiz' and 'fusion-icon' packages and then starting 'fusion-icon' by hand.
The setting in KDE's control centre (Components, Session Manager) doesn't seem to be enough.
Putting the fusion-icon command into ~/.kde/Autostart/start-compiz.sh is a good workaround (with a 'sleep 5' before it to let X start OK).
Even then compizconfig-settings-manager didn't like my settings, so I press the 'restore defaults' button in it's preferences and set things back how I like them (random close effects, wobbly Windows and an over the top desktop cube).