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.
Only if Amazon have off-line ultra HD. LTE is not reliable at some location and insider building. I would do hard wired streaming at home or off-line on the road. Doing away from MQA is good if Amazon can do 192/24 or at least 96/24. My Oppo HA-1 and 105D can't take MQA.
Logged into Amazon Music on Node2i,  HD option shows up as menu item. When you click on it it asks you to sign up