IMHO Apple System Administration doesn't really exist anymore. OS X Server is just an app that helps the end-user setup some services on your home or small-business network. Yes, setting up a DeployStudio Server running DHCP, DNS, NetBoot, AFP etc. seems to need system-admin's knowledge and thinking, and that's true. But the level of support needed to keep those type of services running is very low....
Thats pretty much the situation I have at hand.
Until now a mac mini with external storage has served as a pseudo SBS for us. "Pseudo" because it is a bastardized setup intermixing desktop and server applications. Its main roles are taking care of backups from multiple network clients (all OS X of various versions) which is done via carbon copy cloner (rsync basically), as well as to provide NAS for the office and a MAMP stack for the intranet.
Not a whole lot - but too much for a mac mini to handle cleanly without a ton of external devices attached (we need a lot of disk space). Also, the mini is co-used by bookkeeping as a desktop machine (what could possibly go wrong
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Now granted, this discussion was more centered arround enterprise use and not so much small business - but honestly - OS X server has never been adopted by enterprise for reasons already pointed out. So I think it best compares in the HS/SBS category.
Where to go from here ...
Mac mini Server / Mac Pro Server?
The mini already too much processing power for the tasks at hand, which would not be a problem if there wasn't the price tag to go with it. Also, I am no big fan of having a ton of peripherals for the base setup. This would be more manageable with a Mac Pro based server, but pricing and horsepower are absurd. The other issue I have with OSX Server, is the insane bloat you get with it. Why would a dedicated server need stuff like LaunchPad, GarageBand and the whole myriad of desktop userland applications. Compare that to a minimal install of your prefered distro. On the pro side - the learning curve to integrate a OS X Server is probably the lowest. Although I would expect things to get hairy in case you run into trouble or have to meet non defaults criteria - simply for the lack of widespread adoption and ressources.
We'll be taking things away from the mac bubble (for the server at least) and will replace the mini with the HP Proliant Microserver:
If you compare the pricing for the different options, all in base config with 8TB storage and 16GB RAM based on retail pricing (no apple BTO), it is a no brainer (and the mini is not as "cheap" of a server as it would seem):
Mac Pro: ~3500 (3099 base)
Mac mini: ~2000 EUR (1029 base)
HP Proliant Micro: ~600 EUR (170 base)
Granted, it will take me some time to get everything to work smoothly and if I didnt find this stuff fun the cost advantage would likely be eaten up by the time investment - but - this setup will get me a nice clean little box running 2 VMs on ESXi (a pitty apple refuses to open up here) that can easily take care of the required services with a form factor that trumps anything apple can offer for this use case and will make scaling up easier.
Number crunching tasks aside - I dont see where the mini server gets to justify the steep price and awful ergonomics (for a server box that is).