Niche
The Duo is not a new Microsoft concept. Microsoft Research developed a concept called the Courier (a Windows tablet with two 7" screens) in 2009 and even before that the Codex prototype (two OQO Windows tablets with 5" screens). The idea in Microsoft's mind has always centered on productivity in Office applications.
Even as an early adopter of Windows tablets (pre iOS or Android), I can only think of them as niche products.
They are definitely overpriced and underpowered but if you need the features they are offering you will accept that.
OTOH, I don't see normal people (or even the El Reg community) buying many.
Biting the hand that feeds IT
