Reliability and uptime, in our experience with RHEV, are all based on Linux, and the next version of Linux will have that in-memory upgrade capability, so you can literally upgrade the kernel without rebooting. For now, it's similar to its competitors – if you're upgrading a machine, you just vMotion everything off of it, upgrade your one node, move them back, clear the next node, and so on. But in the very near future, you won't even have to do that. You'll be able to do the in-memory upgrade. I haven't seen any major issues with stability. Ten being the best, I would rate Red Hat and VMware both at a nine and Microsoft at an eight.