My desktop was running on a 640aaks. It has been fine. The drive was supposed to compare to raptor drives and it provided enough space to keep lots of stuff on the desktop. Having access to a matching pair of Velociraptor drives I decided to take a stab at RAID 0 with them and pull some extra performance out of my desktop.
Performance about doubled. Totally worth it. It does introduce another layer of complexity and another point of failure, now if either of the two drives fail then I lose the array but my home server is still providing nightly backups so I'm not really worried about that.
I lose a significant capacity of local storage, but as I'm trying to pull all my data on the server I think this will not hurt me too much. I was able migrate all of Wife's account using Migwiz, which was great; the transition was pretty easy for her. Migwiz does not transfer apps or app settings such as her Chrome profile, but I was able to manually copy that over.
The old 640aaks is now my WHS backup drive which has the capacity to hold all the backups. As today is World Backup Day, that is exactly what I will be doing to my WHS.