Both Mac and iPhone apps are really nice and look great.
Except for the Now Playing screen on the iPhone. When you swipe up from the Now Playing miniplayer (along the bottom of the screen) on the iPhone, it would be great if that featured the album artwork (like in Spotify, Music.app, Rdio, etc.) with the additional functional controls in the bottom half of the screen. There's a lot of wasted space in the current iteration and the UI does not fit with the rest of the really beautifully designed app.
Also, in Album view, Is it possible to move the text/Play button to the bottom of the artwork or just below it. As it is, the overlay fades the artwork and is not as bright and vibrant, taking away from the artwork.
You never get a really good view of the artwork, except for in the Library view - I'd like to see improvements in the Now Playing and Album views, thank you!