Love that Server is now OS-independent, and that the version I own now will work perfectly on El Capitan. Previously, a version of Server that ran on, say, Mavericks, required a paid upgrade to a version that would work on Yosemite. Server 5 appears to have eliminated that.
So what's the big deal about $20 a year? Most corporations can swing that, right? Well, I'm not a business…I'm a home user. So why do I use Server? Read on, and you might decide to use it yourself.
My household has three Macs: my mid-2011 iMac (which is running Server); my mid-2012 MacBook Air; and a late-2007 iMac. There are also two current iOS devices: my iPhone 5; and an iPad 2. In my setup, Server's caching server is a godsend; I download an app or software update on one device, and, so long as the update is compatible, other devices are able to use a cached download on my iMac instead of having to fetch it from Apple's servers again. Even though I have a reasonably fast internet connection, the caching server seems a lot faster than redownloading the app or update. As well, this helps with bandwidth concerns; if your connection is metered and you have a lot of devices to update, caching server should prevent you from using too much bandwidth.
Another great feature is centralized Time Machine backups. Now, in some cases, an AirPort Time Capsule may be a better option, if, say, you didn't want to leave a Mac on all the time, or you want to upgrade your network to a fast 802.11ac connection. However, if you don't want to spend the money on a Time Capsule (for example, if your networking hardware is good enough, or you've got ample storage on an external drive already connected to a Mac), Server facilitates setup of a centralized point for Time Machine backups, appearing much like a Time Capsule would. Each of the Macs in my household does have its own external hard disk for backups, but I do like the redundancy of having a second centralized backup on my 2 TB external drive connected to my iMac. Backups aren't as speedy, of course, but I'd say they're about comparable to the speed of my cousin's 802.11n Time Capsule. (Also, for the longest time, I couldn't figure out why my iMac would wake up in the night, go back to sleep, and repeatedly wake and sleep again; turns out Power Nap was enabled on my MacBook, and it was trying to back up to the sleeping iMac, which was kindly waking for network access. Problem was, it was waking *me* too, so Power Nap got turned off.)
The only problem I'm having with Server is that I can't seem to get NetInstall working properly. NetInstall, if it worked properly, would allow me to have network-bootable versions of OS X, as well as OS X installers and a back-to-basics restore image. It probably has something to do with the network infrastructure in my home, but I simply can't get any of the other computers to boot from a NetBoot, NetInstall, or NetRestore image.
All in all, even home users can benefit from having Server on a desktop Mac. Even if you don't use anything else, the caching server and Time Machine server can prove indispensable for a home setup.