I recently upgraded my XenServers from  4.01 to 4.1 release. Overall the upgrade is easy. Simply pop in the CD and follow the instructions.  All of virtual machines are preserved after the upgrade. It's a good practice to back up your vms before upgrade though.

Here are a few observations/tips I gathered from my upgrade exercise.

If you have multiple XenServers in a pool, I found that you may need to upgrade your master server first. The general tab of the server property will show you which server is the master server in the pool. I upgraded my slave machine first and couldn't use it without also upgrading the master machine in the pool.

Use the XenCenter that comes with XenServer 4.1 to connect to the upgraded servers. Older version of XenCenter may give you incorrect error messages.

For some reason, after upgrade, some of my windows virtual machines had one additional NIC added. As a result I couldn't boot them up. Removing the extra NIC solved my problem.

The new version of XenCenter has some nice small UI improvement. For example, it shows CPU memory usage on the general tab now.

Don't forget to upgrade your xenserver tools. Without upgrade, XenCenter won't display the ip address on the network tab for example.

I hope you find the information useful and I'd like to hear your experience with XenServer too.

Here is a screen shot of the new XenCenter.