Q Which should we buy for our group of 11 designers to use as a central file server: a NAS box, a low-end Mac Pro with a daisy-chain of Thunderbolt drives, or a Mac mini running OS X Server?
A Hitting the right balance between cost and function is not easy, but given that the Mac mini with OS X Server is both capable and relatively cheap, you might find it the overall best option.
Network storage (NAS) systems are widely available with many different capacities and costs, but most are fundamentally Linux servers that support Mac (AFP) file-sharing. As they do not run OS X itself, sharing may not always be seamless, and they cannot run OS X Server, so will always fall short of its full set of features, even if some offer more than just file sharing.
A Mac Pro running OS X Server provides complete compatibility, but you also pay for a lot more; without a copy of Mac OS X Server its services would be limited to peer-to-peer sharing. Although Thunderbolt drives and enclosures perform very well indeed, they remain relatively expensive. However a four-slot hardware RAID system could be extremely potent.
A Mac mini with OS X Server is as easy to configure as Apple claims, particularly if you only use file sharing and a few other services. Sadly the previous Server version with two internal 500 GB drives is no longer available, so you would probably need to attach additional external storage. This could also suffer from the premium prices charged for Thunderbolt peripherals.
Although not the fastest Mac in the world, for a few dozen users it is likely to cope with far more than just file sharing services if you wish, and costs little more than a NAS of reasonable quality and capacity. Like a NAS, it also runs happily without a monitor.
Updated from the original, which was first published in MacUser volume 27 issue 14, 2011.