I've been dying to upgrade my MythTV setup to use Lenny for ages, ever since I got a HD HomeRun and discovered my primary MythTV front end couldn't play back the HD recordings for some reason (I just got a blue box and the audio, and a lot of errors in the X server log).
I figured it was either the kernel or X, and I hadn't been able to get the lirc kernel module to build against anything newer than Etch's 2.6.18 kernel (as I discovered today, that was because of some contamination in module-assistant's build directory, so I could have tried a newer kernel ages ago - drat), so I was unable to determine if it was the kernel or X.org itself.
An added bonus of upgrading X was it talks to the TV better. I've got the PC hooked up to the TV's VGA input, and previously, the best I could do was get it to talk 1024x768, with the TV set to a 4:3 aspect ratio. When I initially upgraded, for whatever reason, the TV was very unhappy with what it was receiving on the VGA input and wouldn't display anything, so in desperation, I tried ditching the xorg.conf entirely, and then everything just worked. X came up with a 1280x768 resolution, I think despite the TV thinking it's getting 1024x768, so now we can have the TV in 16:9 mode, and things don't look all stretched, even when the playback is only 4:3.
And HD, oh it looks glorious. We just watched the Oscars, and it completely filled the TV and looked absolutely fantastic. Playing back HD content seems to really peg the CPU though. The playback was unbearably choppy until we paused the commercial flagging job that was madly trying to flag commercials in the same recording we were currently watching (i.e. the Oscars).
The only thing that seems to be acting a bit weird is notification pop ups from the D-Bus notification daemon. I've got a Griffin PowerMate that runs a home-grown script to re-pair the Bluetooth keyboard and mouse, and that uses the D-Bus notification system to provide feedback while it's running. It seems that while the MythTV front end is running, those pop ups aren't displaying correctly. I haven't gotten to the bottom that one yet.
I also had some problems with a couple of init scripts and splashy causing bootup to hang, but I'm not sure that's completely reproducible (I don't reboot this machine all that often) so it may have been operator error.
Overall, very happy with the outcome. The upgrade was fairly smooth, and I've got the additional functionality I've been waiting for.