Is anyone using KVM in production?

We've had several KVM hosts up and running on Ubuntu 9.04 and 9.10 for the last 6+ months with some minor issues while initially setting up guests, but none since. Our hosts average 3 guests each and the guests do lots of different things, from multi-homed VPN servers to NAS systems with direct access to hard disks. The performance and ease of setup (once the learning curve was out of the way) has been transformative for our network.

Migration has been very easy through VNC connections over ssh via a virt-manager client; cloning has not been as smooth, as new instances have had a tendency towards flaky network errors.

The only other ongoing complaint I have is that it would be nice to have support for QEMU VLANs as an option in virt-install so we could avoid having to use the more tedious KVM commands directly. Other than that, we've been extremely happy, and most of the manangement tools are shared with Xen so I'd say use whichever one works best for you.

We've been using KVM on Debian Lenny for almost a year now.

Stable, except for live migration. Apparently that would work without paravirtualized network. But we can live with short breaks until a patched version becomes available.

We use DRBD to provide shared blockstorage.

Nothing at the time provided a nice way to administer our specific combination so I reinvented the wheel: http://github.com/ptman/kvmmgr