I am using Xenserver 5.
Citrix has released XenServer 5.5 following is feature list.
live snapshot and clone operations on all storage types.
Active Directory integration.
Workload Balancing to optimize.
support for Red Hat Enterprise Linux 5.3, Novell SLES 11, and Debian Lenny.
XenCenter improvements.
Here are some ready made templates free for download and use for xenserver 5.X
Ubuntu 9.10 64bit PV for xenserver
http://www.isaaczarb.com/ubuntu-9-10-x64-template-for-xenserver-5-x/
Ubuntu 9.10 32Bit PV for xenserver
http://www.isaaczarb.com/ubuntu-9-10-for-citrix-xenserver-5-x/