I was fairly positive in my review of the 2010 version of Apple's smallest server, and fortunately, I can say that it's very easy to be happy with the 2011 Mac mini with Lion Server.
The Mac mini server is not a big enterprise server. It's targeted at the small office/home office market and small and medium businesses. As such, it does not have redundant power supplies, hot-swappable components and other features found in enterprise servers. Then again, for the same price as Apple's discontinued Xserve enterprise server with the proper specifications, you can buy several Mac mini servers to get redundancy. Apple does offer a Mac Pro with Lion Server if you want the heavy-duty hardware.
Apple upgraded the Mac mini server's processor to an 2GHz Intel quad-core Core i7, which is a major upgrade in CPU power from the 2.66GHz Intel Core 2 Duo in the 2010 model. Apple also increased the overall memory bandwidth; the Mac mini server uses 1333MHz DDR3 RAM, an improvement on the 1066MHz DDR3 RAM in the 2010 model. The new processor and faster system bus add up to a box that can handle rather serious data transfer speeds when configured properly.
I tested the standard configuration model of the Mac mini server with 4GB of RAM and a pair of 7200rpm 500GB hard drives. Let's compare this unit to a build-to-order (BTO) 2011 Mac mini with Lion Server equipped with 8GB of RAM and a pair of 256GB solid-state drives. In my limited testing, I performed file transfers of a 2.4GB disk image over a gigabit ethernet network, using a 17-inch 2.2GHz Core i7 MacBook Pro with a 5400rpm hard drive.
The network results aren't a surprise, since all tests are done on the same network. It's easy to see that network bandwidth is more of a limit on a Mac mini server than internal memory/bus bandwidth or the storage devices.
The BTO Mac mini server's internal transfer speed shows a significant jump in performance, almost double, for just over twice the cost of the standard configuration model. Reboots during updates happened so fast that at first I wasn't sure they'd actually happened. Just for fun, I started pinging the server, and then I rebooted it. At most, I'd drop two packets. SSDs are fast but not cheap, and many folks may not want to pay the extra for them, though the performance increase is large enough to make the overall value pretty darned good.
The other major change with the Mac mini server is the new Thunderbolt port that replaces the Mini DisplayPort. The Mac mini still has a FireWire 800 port, four USB 2.0 ports, an HDMI port and an SDXC slot. Not only is Thunderbolt much faster than FireWire 800, it's an adaptable bus, even outside of using it for a display. For example, you can use a SANLink Thunderbolt to Fibre Channel adapter from Promise, and plug the Mac mini server into a Fibre Channel network that you'd use for, say, Xsan and StorNext, or to use the Mac mini server with existing Fibre Channel storage.
So now, you can have a Mac mini server hooked up to high speed storage either directly through Thunderbolt or through a Thunderbolt adapter like the SANLink, and still have the gigabit ethernet port free, which means that server and storage communication isn't affected by other network traffic. It's a nice step in the right direction.
As utility servers, Mac mini servers are quite awesome. In the past, I've used them for Open Directory servers, security camera software servers, remote access servers, anything where I need a dedicated server for some task but don't want to spend a lot of money and need it to fit into my existing infrastructure. With the latest model, I've added a file server and a network home directory server to that list, and the reason is simple: Thunderbolt.
One of the past problems of the Mac mini server was that you had only one high speed interface for storage, the gigabit ethernet port. Yes, there's FireWire, but there's not a lot of managed storage devices with FireWire ports. If you do have a storage device on FireWire, you have to have a dedicated application on the server to manage the storage. And FireWire is not common on remote storage with SNMP management.
It's not all peaches and cream with the Mac mini server. I'd really like to see Apple start using ECC memory or offer it as an option, it provides real peace of mind for a server, without a huge increase in cost. I also wouldn't be terribly upset if Apple were to add more hardware monitoring to the Mac mini server's SNMP implementation, such as temperature, voltage and so forth. These are small things, but important.
Can you replace a high-end server with Mac mini server? No; if you absolutely have to have a rack-mounted server in the Xserve vein, you're not going to be happy with a Mac mini server.
But these are not showstoppers. In fact, the only major weakness of the Mac mini with Lion Server is the OS itself, which as I noted in my review of that product, is a good start on a major revamp but has some serious issues. Unfortunately, because of the hardware changes, you're stuck with Lion Server on the Mac mini. I have seen folks get the new Mac mini server to run Snow Leopard Server, but it's pretty involved, and not something I'd ever recommend. From my point of view, the excellence of the Mac mini server is worth it.
The Mac mini with Lion Server is a real improvement on an already solid performer. In its target market (companies with up to 200 users), it is an outstanding performer and is also a capable utility server in a wide range of usage needs. The Mac mini server is not a good choice if you need hot-swappable components or redundant power supplies in a single box, but the Mac mini server's cost allows you to achieve redundancy in other ways. While the standard configuration is an attractive setup, consider paying the extra money for SSDs, since they add considerable performance for the price.
The addition of Thunderbolt to the Mac mini server, as well as some timely third party products has increased the capability of the Mac mini server far beyond that of the previous version. The lack of ECC RAM and a somewhat limited ability to remotely monitor hardware conditions are notable, but they should not stop anyone from implementing a Mac mini server. Lion Server has some issues, but as they're fixed, the Mac mini will become an even more excellent server.