I've been running a WHS server for several years now (did the beta then bought the product at RTM); it's on an older Dell desktop that I added 16 1TB drives to via 2 controllers each with 2 eSATA ports (using Addonics port multipliers to connect the drives).
The WHS backs up the various PC's in my house (4-8 depending), and acts as a file server, print server, and media server to a slew of devices. It's really excellent, and I have had very few issues with it at all - it just runs and does it's thing.
The strategy WHS uses to provide reliable storage is very interesting, sort of an app-level (folder/file-level) RAID-1 approach. NTFS reparse points are key to the strategy and it is both simple to manage and very effective. A key benefit versus RAID is that with all the drives managed as a large storage pool, the system can be fully immune to multiple drive failures (and if not fully immune, you will only lose a relatively small amount of data on successive drive failures).
Performance of WHS has been excellent as well; my system (with two gig-E network ports) can serve lots of clients at once, including sourcing multiple mpeg-4 video streams. The storage approach in WHS makes the most of slow disks/controllers, and uses idle time to do the leveling/replication.
But regardless how effective WHS is, it's no substitute for an off-premises backup solution! I've been doing the take-a-disk-to-work thing (Vantec EZ-Swap cartridges rock), but that's getting old so I'm looking for a net backup solution for WHS itself.
In short, it's been a great system for me....