No point in WHS without DE
I've recently replaced a 5 year old HP rack with a Tranquil PC WHS box - mainly to save power, the fact its built on Win2k3 just meant I knew I wouldn't have s/w issues.
Now I have NAS boxes which do RAID 5 and that's fine in general but when a drive fails it obviously takes a fair old while to rebuild the array - during which time the contents of the array are pretty much inaccessible (for writing anyway).
With drive extender however its a hell of a lot faster - I'm actually very impressed with it much to my surprise. The tl;dr for anyone wondering what DE is - basically its a mirroring system for spanning multiple disks (my WHS box has 6, although two are in a RAID module).
DE has a load of issues with any sort of defrag (even MS's defrag) and some AV programs throw wobblies about it (NOD32 is fine). However its stunningly simple to use - simply replace the drive that's failed and DE will rebuild the storage pool. Even works on hotswaps. The average user doesn't need to know anything about admin stuff - all he needs to do is add the replacement drive to the storage pool using the Janet & John console.
Without DE then there's no point in WHS. You may as well just opt for the full server product or build a box with Win7/Linux and some RAID.
HP know the score and so do MS. Vail is dead in the water without DE.