I know this sounds nutty, but after having lived with a now-stable NAS solution (Buffalo Terastation) for about 6-months, I would state that the preferred solution for network storage should be a true dedicated server with a raid array. This probably adds $500 to the initial investment, but the flexibility, performance, and duarability is, to me, worth it. That will be my future architecture.
I am starting to come around to what everyone has been telling me - the commercial pre-packaged solutions make too many compromizes versus what you can build "roll your own" for the same money.
Especially if you have a team of skilled network engineers (like I do) at work that can help you set everything up. On the downside, these folks are trying to talk me into a SAN because "it rocks." They are used to spending other people's money.