I also have to agree. I had used MOG (and put up with their biggest downside, no gapless playback) because it had the best sound quality among the various rent-a-library streaming services. When it got replaced by Beats, I gave it a try, but was unimpressed with the concept, the app, and, well, it's Beats - need I say more?
Since then I've been using the (rather awkwardly named) "Google Play All Access Music". The quality is usually excellent, the library is on a par with other similar services, and I can upload my own music too. And gapless playback is almost perfect, with only the slightest hiccup between tracks.
While some people don't care much about gapless, for anyone who listens to live recordings, Progressive (or "Art") Rock or Classical it is a must, as tracks often flow together, and having a several-second pause between tracks simply ruins the listening experience.
As for Amazon Prime Music, I figured I'd give it a try since I'm paying for it anyway. I was thoroughly unimpressed. As ebann already pointed out, there s no simple discover-and-play capability, at least not via the web browser (anyone tried the Android or ios apps yet?). You first must find the music, then add to your library, then go find it in your library, then play. It's tiring just to think about. Oh, and the aforementioned gapless playback? Non-existent. In fact, it seems an extra-long pause is added. My guess is it doesn't even start buffering the next song until the one playing finishes.
And the selection is poorly lacking. I know they're just starting, and hopefullly they'll get more artists/labels on board, but there are even big holes in the discographies of artists they have. Not only missing complete albums, but, perhaps even more annoying, albums where some, but not all, of the songs are "Prime". So to enjoy some albums in their entirety with Prime, you still have to buy one or more tracks. Ridiculous.
Oh, and the sound quality. While it's marginally better than 128K MP3, it's definitely not as good as Google's (or other streaming services, for that matter). While Google and most others rip everything at (at least) 320-360K, Amazon's music is at BEST 256k(average)VBR or even 256KCBR (according to info on their website). The difference is definitely audible to anyone using decent 'phones or speakers. Perhaps they assume their target audience uses cheap earbuds or bluetooth 'phones/speakers for all their listening?
Anyhow, that's my take on Amazon Prime Music. I know I was a bit harsh -- I suppose as part of the bundled "Prime" services I shouldn't complain, but it is what it is and I prefer not to mince words.
