"If you want to stream music, you’ll need at least a 1.5-2Mbps LTE connection for HD quality and 5-10Mbps for Ultra HD. A large data allowance is a must too – a 3 and a half minute song eats up 51MB in HD quality and 153MB in Ultra HD. A lossy track is 9MB."

Above (posted earlier) verified from multiple outlets. [Quote is from GSMArena]
Pertinent information from Apple Insider:

"HD tracks are 16-bit audio with a sample rate of 44.1kHz, or CD-quality, and an average bit rate of 850 kilobits per second. Ultra HD tracks 24-bit audio with a sample rate of up to 192kHz, and an average bit rate of 3730 kilobits per second. As Amazon points out, most streaming services only offer standard definition quality music, which reduces some of the detail in order to save on file size.

Because the files are lossless, it should be noted that they are significantly larger. A standard song comes in at just around 9 megabytes, where as an HD song comes in at 50 megabytes. An Ultra HD song comes in at 153 megabytes, meaning that devices will be able to hold less music at higher qualities.

This also means that streaming HD and Ultra HD songs will use up more of your data plan, so it's advised that people with lower data caps listen to music via Wi-Fi or download the music for offline listening later.

Additionally, the quality of the music is going to be heavily dependent on how you're listening to it as well. Playing the music through low-quality speakers is going to render a low-quality result."

In a lazy and totally non-scientific comparison across a couple of my 2nd-tier hifi systems:

Amazon Music "HD" and "Ultra HD" quality levels seem comparable to similar-res audio from Tidal and Qobuz (16-bit/44khz and 24-bit/48kHz). I didn’t compare to Tidal MQA. If anyone thinks they can objectively ’hear’ and distinguish hifi audio ’quality’ differences above 48kHz, or if you don’t but really-think-it-matters, read this: https://people.xiph.org/~xiphmont/demo/neil-young.html. <= the irony is poignant, given Mr. Young’s latest gushing about how "Earth will be changed forever when Amazon introduces high-quality streaming to the masses."

Amazon’s web- and Mac/Win native players are consistent with other services’ apps: a race-to-the-bottom in effective UX and functional design. In an equally non-scientific and lazy study I have the Spotify, Qobuz and Amazon apps tied for dead-last, then Tidal at barely better. All of them suck in various ways.

Amazon is still working out issues in their catalog play-back (wrong tracks behind some albums, no tracks behind some "Ultra HD" albums etc). Catalog curation is also kinda ’wtf?’ in places. No surprise - greed/fear/hubris rushes most consumer digital media services to market. What are free trials for, anyway?
So far so good. I upgraded Amazon to HD and it sounds very good. The interface is different than TIDAL but it shouldn't be too difficult to navigate. My stereo is far from great YBA electronic into Vandersteens with a Scott Nixon dac but for CD quality streaming it's not bad. As I get further into it I'll update.
just started my free trial--initially i find the UI inferior to and less intuitive than spotify but better than tidal. catalog seems deep, if not as extensive as spotify. ultra hd sounds great; hd sounds better than spotify, though it's not a night-and-day difference and there's less ultra hd content than expected. playlists are ok, though not as well-done as spotify--i expect they'll markedly up their game and add content as they gain traction, since amazon is very customer-focused and has long-term vision on their road to world domination.