It depends on the usage: Test server or productive system, small work group or many clients, maybe even web clients?
One weak point of the Mac Mini is its laptop-type hard disk that has a high risk to fail with data-intensive tasks. Other weaknesses are missing hardware redundacy (disks, power supplies, network adapters) and failover as a good server would offer.
How much is your data worth to you?
If you are a Technet member search the Technet forums for Mac Mini on discussions on this topic.
Personally, I would never go this route for a productive system. Others mileage may vary.