[I would provide more links but I can’t be arsed to go dig them up again. See my recent twitter stream for some of them.]
1) Ubuntu 9.10 Karmic
All the xen-tools and stuff install nicely from the repository, but there is no Xen kernel provided. None. At all. You have to go wading through Debian documentation and try installing a Debian Lenny kernel, and it all goes horribly wrong. Also Karmic introduces GRUB2, so all those HOWTO documents that talk about editing /boot/grub/menu.lst are now out of date because GRUB2 is a lot more complex (cf: /etc/rc1.d/). The Canonical Ubuntu forums refer to tickets complaining about the Debian Lenny kernels, and that Ubuntu should not adopt the Debian Xen kernels but roll their own. They have not yet done this.
2) Debian Lenny
Installs quite nicely and cleanly. Xen enables easily – install Xen kernel and reboot – but your framebuffer gets eaten so X11 no longer works (and puts-up interactive warnings on screen to tell you so, so you have to be around to press return a few times) – and then the out-of-the-box config does not appear to work with any of the prebuilt Xen images from Stacklets.com. Messages about “waiting for vif.1 to become free” (when you are using NAT?) and that domUs are dying immediately upon creation and without diagnostics resulting, does not lend confidence.
3) CentOS 5.4
Installs quickly from DVD, and “virtualization” (Xen) is selected easily from the installer menu. The resulting Xen kernel panics on first reboot, due to lack of PAE hardware and that Centos has assumed PAE is available. Can progress no further on the hardware available.
None of this bodes well for Xen in my opinion; I know they are owned by Citrix now, so there should be money behind them, and my feeling is that if they want to be more than a niche player in the increasingly commoditised virtualisation market they should throw money and an engineer at Canonical and make Ubuntu work as a turnkey solution.
 I have heard at least 4 people tell me to “use KVM it’s the future” – but (a) I heard that about KDE and (b) that’s not what I am trying to achieve, the point of messing with Xen is to grow familiarity around its supposedly portable technology, not least because EC2 is Xen.