From Wilkipedia; "Raid 1 without parity. Provides fault tolerance from disk errors and single disk failure. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate so long as at least one drive is functioning."
If one drive goes belly up the data will still function. Once you replace the faulty drive check your raid software to ensure that the new drive catches up with the raid, depending on the size of the drives and amount of data it may take a little while.
Hope this helps.