I bought this with the HP £100 cashback offer (Check on the HP site as they seem to be repeating it quite often, but check the model number carefully as it is quite specific- I got my cheque within a month of filling in the fairly simple form) so I paid less than £150. Not sure how HP make money on this. I fitted 2 2TB disks in a mirrored RAID configuration and installed Windows Home Server 2011. All went very easily andI now have all my computers backed up to the server and the important data backed up to Amazon reduced redundancy S3 storage (using Cloudberry BackUp) which is pretty cost effective compared with DropBox.
It is reasonably quiet but you probably would not want it in the bedroom or living room (WHS will only work with a wired LAN connection so it sits in the corner of my hall).
Overall very impressed. The low power CPU doesn't seem to be an issue but I did put an extra 4GB of RAM in.