RAID isn't as fragile as Mark makes it out to be, but it does have it's downsides at times. And Christian is exactly right, RAID is not a backup solution. (well, unless you are doing the 3 disk RAID 1 trick Andy does, assuming one drive is kept out most of the time).

Much like many things, it all comes down to the quality of the implementation. There are some pretty bad RAID hardware controllers out there. And some pretty bad RAID software setups. But with a good setup, RAID can do it's job of ensuring one (or two or multiple depending on the level) failures don't take out a system. RAID remains one of the easiest ways to raise performance of a storage subsystem when dealing with spinning drives. And modern servers changed the definition of RAID to mean things beyond hard drives. I've worked with RAIDed RAM before, in servers that cannot afford any downtime.