I'm a Windows Home Server user (I've got one of the HP EX475 machines) and it's great - it just quietly sits there backing up my data, keeping an eye on my machines' status etc.
However, an important update has now been released - called "Power Pack 1" - which includes a number of important fixes and new features. Note that your system provider may well make this available to you via their own automatic update mechanisms. However if you want it now you can get it here. The released notes can be downloaded here. You can also read the development team's blog here.
One of the cool things about Windows Home Server is the extensibility story. It has an SDK (which has been updated for PP1) and as a result there is already a vast array of addins available, mostly provided for free by members of the Home Server Community.
For me the core feature of Home Server is its backing up of other computers on your home network. It can do a lot more (provide remote access, serve your music as a UPnP server, be a file share etc) but for me that is the crucial one. I was round at a friend's house at the weekend and he was saying how he didn't have any photos of a particular holiday he'd been on because it was when he first switched to digital photography and hadn't had a backup. It made me wonder just how many other people are out there with Gbs and Gbs of treasured photos, videos and other digital assets with blind faith in the infinite reliability of their hard disk. It's so easy to not do but so crucial when disaster strikes. Just remember though - Home Server does not back itself up (that would be a miracle!). It backs up the other machines on your network and does so with a considerable degree of fault tolerance (if you have sufficient disks in it like the EX475 and enough space on those disks to allow the data duplication feature to do its job). One of the new features that have been added in PP1 is the ability to back up and restore the content of your shared folders to an external hard drive.