I've been using WHS for over a year now, having first built a WHS server for my parents, before building one for myself.
The first benefit of WHS for me has been the automated backup of PCs joined to the server. Shortly after I built my server, the hard drive in my old Vista x64 box gave up the ghost completely. All it took to get it back was putting in a new hard drive, booting off the WHS recovery disk, supplying credentials to the WHS and then waiting for it to restore over the network.
As mentioned by David Frautnick, there is a growing community of developers creating new add-ins for WHS, for a good listing of the wide range of add-ins and other WHS related topics, have a look at We Got Served
Another benefit I completely forgot about is that any music/photos/videos you put in the shared folders are accessible from DLNA clients, i.e. networked AV receivers - means you don't need a computer in the living room to listen to your music stored on your server.
And another thing, with a WHS server, you can also sign up for a .homeserver.com subdomain via which you can remotely access your home server and remotely control any PCs connected to the WHS server (if they're on of course).