I set it up for a client to run a mail server along with a Mac mini, all the emails including archives and backups are stored on it.
It is much faster than the previous Mac Pro and internal drive setup used for the same job.
Yes, as far as the Mac is concerned it looks just like a big Thunderbolt hard disk and so can in theory be booted from, we however boot from the internal drive and merely store data on it. It took a long time to build the RAID (as do they all) but it has now been running for some time with zero problems and has stopped the complaints about performance.
We are going to get some more to host OS X network login accounts and home directories.
If anyone is considering it, avoid the new Lacie Thunderbolt RAID like the plague. It uses OS X software RAID and apart from that being slower it means it cannot support RAID5. I also advise avoiding their big disks for the same reason (software RAID), as a comparison Western Digital have built-in hardware RAID0 and 1 in their dual-disk big drives.