Generally I look at virtual appliances as a good way to get a quick proof of concept going. I'm hesitant to do this for production, especially since I'm in an environment with a lot of different virtualization platforms (most VAs assume VMware, which is not always available in a given environment for us).
If I need to evaluate a technology or product in a hurry, I'd rather let someone else do the heavy lifting (config/optimization) at first. If I'm rolling it out to production and expecting it to scale, then I'll invest the time and energy and hair loss in doing it from the beginning.