I'm not able to find a clear yes/no answer to this simple question: does using ZFS in mirror mode double the read performance of the underlying vdevs?
This applies to either individual requests or net performance (i.e. a single read request is sharded across two disks or two simultaneous requests are each served simultaneously from different disks).
Best Answer
If using Linux/FreeBSD (also FreeNAS) read speed is much better
Source: http://open-zfs.org/wiki/Features#Improve_N-way_mirror_read_performance It's hard to tell whether illumos/OSX have that patch which rebalances read I/O to the least busy device, or just improved round-robin.
In any case, mirror improves read I/O. As the other answer mentions, using SSDs usually doubles read I/O, while spinning drives increase performance by less than double depending on type of load.
My anecdotal testing on Linux shows a significant speed increase, especially when reading large files or more at the same time.
(Edited to reflect changes since the answer was posted)