It's actually been smooth running except for a few very slightly minor bugs. Like sound, for instance. Completely broken. I don't know if this affects every sound card, but mine isn't detected with the new kernel. Also, suspend to RAM has a very slight, minor bug. Also completely broken. I left two bug reports, No sound on PowerPC with Jessie upgrade and Suspend fails on iBook G3 Dual USB PowerPC, in case anyone wants to add to the crickets left by Debian's kernel maintainers.
Both of these problems can be dealt with, fortunately, by an easy workaround. If you upgraded from Wheezy to Jessie, you can boot into the previous kernel (3.2.0-4) by hitting tab at the second yaboot prompt and typing in "old". Afterward, sound and suspend should be back to normal. You can also install the 3.2.0-4 kernel from a clean Jessie install by adding:
deb http://security.debian.org/debian-security wheezy/updates main
(that's a space between debian-security and wheezy/updates, not a carriage return) to your /etc/apt/sources.list. After installing, follow the instructions on the sixth post on this Ubuntu thread about editing yaboot.conf to tell it to boot a specific kernel.
I suppose those problems will be eventually fixed, but with the latest update I saw even worse news. Xserver-xorg-video-radeon was updated to a KMS-only version, meaning if you have a Radeon GPU and don't have KMS activated, it throws it back to the fbdev driver, meaning video playback will suck and you'll only see 8 bit colors. You can fix the 8 bit colors problem by passing the yaboot parameter "video=radeonfb:1024x24-32@60" at start up (replace 1024x768 with your native resolution). This is familiar to anybody who followed zen's guide to installing Lubuntu. However, this won't help you speed up video playback which, as mentioned, sucks with fbdev.
You can always activate KMS with the yaboot parameter "video=radeonfb:off", but there are downsides (along with the upside of enabling 3d). First, KMS breaks suspend on PowerPC and I found no indication work is being done to correct this. Also, your keyboard brightness keys may not work. But worst of all, when I tried enabling KMS on two of my machines, I got a black screen (iBook) and persistant system freezes after boot (Sawtooth). So KMS is totally unusable for me. So how do I get back my fast 2d desktop with decent video playback that I had with the old radeon driver?
(UPDATE: You can greatly improve window-dragging performance by running Compton ["compton -b" to run it as a daemon])
sudo aptitude updateto update the repositories, then switch to a console and kill X with
sudo /etc/init.d/yourloginmanager stop, and then (on one line):
sudo aptitude remove xserver-xorg-video-radeon
Aptitude told me it needed to remove two dependencies, xserver-xorg-video-all and xserver-xorg-video-ati as well, so I said fine and proceeded. Next, I reinstalled the drivers using the old wheezy repositories with (one line):
sudo aptitude install xserver-xorg-video-radeon xserver-xorg-video-all xserver-xorg-video-ati
There's one more step and that's to put a hold on the radeon driver, that is, to tell your package manager to keep it at that version and never upgrade it. I found the instructions for that at Not So Frequently Asked Questions, but it's basically:
to make yourself root. And then (on one line):
echo xserver-xorg-video-radeon hold | dpkg --set-selections
and to confirm the new setting:
dpkg --get-selections xserver-xorg-video-radeon
Finally you'll want to revert back to the Testing repositories by restoring your edits to /etc/apt/sources.list and running
sudo aptitude updateagain.
Afterward when I did a full-upgrade to upgrade all my packages, the video-all and video-ati packages were upgraded, which I didn't care about, but the radeon package remained at its old version. Now when you startx or reboot, your desktop should be back to its old snappy self.
I should note here, you should be cautious about putting holds on packages. It's possible you can have a cascading amount of packages held back from upgrades as the dependencies on that original held package build up. But in this case with the radeon package it hasn't been a problem, but it's something to keep an eye on. I just made sure to save a note about how to hold and unhold packages from the above link.
I didn't bother downgrading gtk-3-0 because it had too many dependencies. It would've gotten too weird.
Looks like this bug is fixed.
So that's where Jessie is. I admit to being discouraged and depressed about it, especially on the graphics side. A few years ago I had this vision of Linux on PowerPC always getting better and better, but with support for older graphics cards being dropped left and right, and now this KMS-only business, it looks like we'll be patching together systems with sticks and chewing gum for the foreseeable future.
Maybe the Debian team can be convinced to realize they found perfection with Wheezy and to maintain it with security updates and backports for like the next ten years, or at least until our PATA hard drives burn out ;)