I love the xServe RAID. It has been up and running for over 40 days now. It mounts instantly, and is more than fast enough for any uses I have. I use it for graphics files, audio, and general storage. It seems to be very well built.
I bought the full boat. 14 x 400 Gig drives. There are two separate banks of drives. I have 6 drives on each side set up as a RAID 5 and the remaining drive on each side set up as a "hot spare" . That means that I have about 1.81 TB per side of storage. I can lose two drives per side without losing data. All other components are redundant.
My brother has an xServe RAID in a mixed Apple/Windows environment, and says that it works great. I only have Apple computers.
It is loud (though not much louder than the LaCie Terrabyte drives!). You could deal with the noise in two ways: first, you could place the RAID in a separate room, since you can use optical cable as a connection. That is what I have done. Second, you could house the RAID in a cabinet specifically designed to be quiet, available from Apple. Another consideration is the heat generated. The Apple site says how many BTUs it sheds.
The xServe RAID does not come in a cluster version, that is only the xServe G5 Server.
The only downside that I see to the xServe RAID is that it will probably be pretty much worthless in 5 years, as alternative storage systems are developed. Gotta love the computer industry!