"Perfect server for smaller all-Mac environments"4.0 starson by talmy
Pros: Small, can put on shelf.
Very low power consumption
Reasonably easy setup (but not as easy as the client OS).
Easily upgraded to 8GB RAM
Thunderbolt promises fantastic high speed external drive expandability
Hostorically most reliable Mac in lineup
Cons: Lion Server OS is inferior to Snow Leopard Server OS
It would be nice to have a second FW800 port and additional USBs or even USB 3 ports
SD card slot, HDMI and dual monitor support are not appropriate for a server system -- space better used for above po
Summary: I'm currently using an older Mac mini with Snow Leopard Server, which has been running 24/7 for a year and a half with no problems, serving DHCP, DNS, file sharing, TimeMachine, calendar, addressbook, LAN wiki, VPN, and Windows Quicken via a virtual machine to a home network of 6 other Macs, a Windows PC, and assorted iOS devices. It has 4 1-2 TB drives attached via FW800 and USB. The new model is better in every way (but one) and I can recommend for anyone in my position of needing a server for a Mac environment.
My only real concern is Lion Server OS. I've tried it out on a development system and found many features either missing or with reduced functionality. It looks like I could do the upgrade successfully but as it offers no new features I won't be doing so. However anyone considering this system needs to look at its functionality carefully -- it no longer uses Samba for SMB making its use in a Windows domain questionable.