Earlier quoted context omitted.
It's fantastic. Either it'll eventually take off, or else either Apple or Google will copy all the good ideas and I'll switch back. Either way works for me, frankly. (Well, no, that's not entirely true. I'd prefer it take off next year and Nokia survives intact, because nobody else is making hardware that isn't hopelessly bland looking.)
What's fantastic about it? I have a new Lumia (I make apps) and it's really not very great. The hardware is nice, but the software... It looks nice at first but it happens to annoy me no end compared to iOS / Android after a while. I still cannot put my finger on it what that is. It's also massively instable for me; someone here told me that must be hardware issues so I had it replaced; same issues. 3 out of 5 times…
About space inefficience, with Metro I can see at a glance the most important things. You know, less information density -> more focus.
For me, the best features are fluidness (I still see last generation Android phones which are laggy), the live tiles are great to have all the information at a glance, and I really love the Metro design. Also, the deep integration with Twitter and Facebook makes it a really good system.