Yes you lose space, but not speed.  I've read you may actually have a slight gain in reed speed, but that's irrelevant.  The difference is it's a mirror, not a backup, so if you delete something it is deleted from both hdd's.  However, you have the benifit of being able to replace a drive if it dies while still using the computer.  It will be slower while the RAID arrays rebuilds, but in a business the lack of downtime is important.

@RoadStress: I don't mean to ignore you, I just don't know the answer to your question.  I would love to see a seperate topic started on this.  So if you start one, please link to it in this thread or send me a pm.