UPDATE: Since I originally wrote this, they have since added everything in the "What's missing" list except for browsing a genre. Google All Access Music seems to do this better but I am still loving Spotify due to the integration into the iPhone and my Onkyo receiver at home.
I finally sank my teeth into the now US enabled Spotify music streaming service. My initial impression is that Spotify will soon make all other music services irrelevant especially with a few small tweaks.
Let's start with what they got right:
1) Access to basically any music any time you want.
2) The ability to subscribe to your friends play lists and even collaborate on play lists
3) The ability to directly recommend a song or album to a friend
4) How easy it is to find new music and listen to the full album right away
5) Synchronize everything between your mobile device and your desktop
What it's missing:
1) The ability to choose a song or artist and generate a smart playlist that will contain a variety of music like what you have chosen
2) The ability to pick a genre and listen to it as if it where a radio station
3) The ability to browse genres to find music
Some nice to haves:
1) Multilevel grouping of play lists
2) Some sort of conversation element between friends
3) "Subscribing" to an artist to be alerted of upcoming album releases and concerts in your area
If the missing items come to be, it will instantly render all other music services useless and irrelevant.