Just some tidbits of real life happenings:
I started out with a RAID 1 (a mirror) on one of my Linux machines. Bought a couple of new drives, and did not have enough SATA ports. I created a RAID 5 with two drives, copied over all the stuff from the RAID 1, took out the old RAID 1 drives, added two new drives to the RAID 5, and now have three active disks in my RAID 5 plus a single hot spare for my system, in case problematic things happen.
It did take some time, but it was a straight forward process.
With LinuxMCE and the changes jondecker did to the RAID part of the system, things like managing RAID 5 within LinuxMCE became much easier. Still, as both merkur2k and Zaerc have pointed out, it is not a backup solution. For me, RAID 5 is a solution which keeps my system running in case a single drive failure happens, so I can work a bit longer, get replacement drives etc. For data I have a backup, for media data I do not.