Usually I'm not too thrilled about Amazon taking over markets, brick and mortar, etc.  And I'm no expert on the economics of this.  That disclaimer aside, it does seem like it will put market pressure on all to stream higher resolution and that is a good thing for the hobby. 

Off to check out the terms. 
I've signed up now trying to figure out how to get it to work on my system. I'm using JRiver on my laptop and bubbleupnp on Android tablet. I'm going load the Amazon music app on to the laptop.  If anyone is successful please post instructions. 
Yes.  Like being able to control the Amazon music on your main computer with an iPad.
I’ve been using Amazon Music Unlimited, and pay $79/year. I control the program with my phone.

HD is $5 more a month.

Free trial for current and existing customers. I’m trying to upgrade now, but the webpage is very busy according to customer service.

I’m looking forward to trying it out
So download Amazon Music on your iPhone or iPad and you can control the new HD Music on your main computer?