So I'm personally not quaking in my boots at the competition with this round of releases...
SUSE 10.1 won't install in VMware. It just won't. First it refuses to believe there's any software on the DVD ISO (which VMware shows it as a DVD drive), then you can work around that by dropping to the text interface and validating the DVD first, but it then coughs up its guts trying to format the swap partition. I'm rather surprised that they didn't, it seems, test the darn thing in VMware before releasing it - it's a fairly common use case. (Google suggests I'm not the only one encountering these problems).
Ubuntu 6.06 - the 5.10 to 6.06 upgrade process went quite smoothly, they have a nice little in-place update system within the software update tool, but it inexplicably failed to update gcc. Or the kernel headers. Both of which I had installed. This meant that on reboot, X broke (because the VMware mouse driver wasn't there any more) and the network broke (because vmxnet wasn't there any more), which I'd expected, but I couldn't rebuild them because the tools weren't there. I went back to the default mouse driver and got X.org going again, then had some trouble restarting the network - Ubuntu's network config tools don't appear to be up to much, they just no longer believed the connection even existed although all its configuration was obviously still there, because when I manually loaded the pcnet32 module, the connection popped up fine. Seems the config tool will only work when the connection's working, which is somewhat...pointless. I was then able to manually update the kernel headers and gcc and get things back in shape, but it's a bit messy, really.
Ah well, back to the coal face.