A little while ago I wrote athat touched on the usefulness of the Mac Mini as a server. Since its release, the new mini has shown solid sales and clear interest in a small server product from Apple. Recently, Ars Technica has posted an in-depth review of the server software running on the new Mac Mini, attempting to in part define the exact niche for the system.
Check out their review here: http://arstechnica.com/apple/reviews/2010/01/mac-mini-with-snow-leopard-server-review.ars
Overall, Ars Technica's review mainly covers the features of Snow Leopard server which can be run on any modern Mac, but with regards to its use in the Mac Mini they point out two drawbacks in the new design: lack of official support for 8GB RAM (though it can be done with aftermarket upgrades), and the use of 5400RPM hard drives with no 7200RPM option. In most heavy computing cases I would agree with them regarding these hardware limitations; however, in cases where these features are required, perhaps the Mini would not be the best server solution.
For most home and small business uses, a 5400RPM drive will provide perfectly adequate transfer rates and access times for services, media, and files. Since as a server the Mini will not be running games, video processing, or other applications that demand high performance from the hard drive, there would be little noticeable benefit to having 7200RPM drives for the Mini. Despite this, I agree that the price difference between 5400 and 7200RPM drives is small enough that Apple should consider adding it in as an option.
The only major drawback I see in using the Mini as a server is the lack of easy access to the hard drives in the event of a failure. Being a server, regardless of the workload on computer, the system may be depended on more than an average desktop (especially for small businesses), and therefore be required to run at all times to provide both network services and files to people.
If a drive fails on an XServe, you can easily swap it out for another one and restore a backup to it (or better yet, have the hard drive be rebuilt in a mirrored RAID array without the system going down). With the Mini, this cannot easily be done. Along with this limitation, Apple provides the system with two large 500GB internal hard drives, and while this may appear beneficial for servers at first, storing common files on these drives may be less secure than using an external storage solution. If a hard drive crashes, both the OS and all the files on it will not be accessible and the system would need to be disassembled for drive replacement.
To tackle these problems, it would be better if Apple supplied the server with easily-accessible drives in a mirrored RAID array (preferably with a dedicated hardware RAID controller) and encourage users to store files on an external firewire drive.
Given the relatively low workload that the Mini is designed for, despite these drawbacks the likelihood of a drive failure is less than that of larger server solutions where the drives are constantly being accessed. If properly backed up the Mini should work just fine as a server without drive failure being a common problem. Additionally, if properly backed up the system should be easily recoverable by just replacing the hard drive and restoring. Perhaps the ready availability of full system backup solutions (Time Machine, Cloning, etc) negates the need for easily accessible internal drives, but being able to easily replace the drive still would be an appealing feature to have.
For more information on the new Mac Mini, check out this CNET review that contains both benchmarks and analysis: