What is the current status, outlook, and pros/cons of ARM processors for high performance computing and general desktop use?

I hear a lot about ARM (and other non-x86 architectures) being the next big thing for high performance computing. Unfortunately, all the easy google searches return with what seems to be pure marketing hype devoid of details, and I've never seen such a machine in use.

My background is in high performance computing, both with GPUs and large traditional linux clusters.

Are there any functioning ARM servers/desktops a consumer can currently buy or build, install linux (or other OS) on, compile C/C++ code on, and execute on? (I see some chromebooks use some kind of ARM CPU, is that the extent of it?)

Are there currently any advantages of ARM by some metric, e.g. flops per dollar or flops per watt?