Apple released Lion a few weeks ago. We upgraded our systems to the latest stable version 10.7.1 and upgraded accordingly filemaker server 11. The upgrade went pretty smooth although we had some manually corrections to make. The interface of Lion is pretty similar to previous OSX systems, in the version (server) we installed we really liked the new centralized server management tool. The only thing we didn't like is not having an easy way to reinstall Lion since it came with our new server and the only way to re-install apparently is via internet with Apple servers. We hope in the future Apple would release a way to format an USB key with pre-installed Lion server on it.

If you are thinking on upgrading to Lion, check the software you use is 100% Intel. All PPC applications will stop working. Also make sure you have a tested backup of your whole system before you install this might be the only way out in case you need to downgrade. You can't re-install Snow Leopard and use migration assistant to port a recent Lion server back into Snow Leopard.