Not that there was a ton of concern to the contrary, but Spotify appears to be the technical feat that everyone claims that it is. This isn't too surprising, since they've been refining the service in Europe for several years, and the version we see as the new, hot thing in the U.S. is actually the result of many iterations.
But how does it compare to other services? Well, it aims to be your everything, everywhere service: streaming music, the music you own, everything in one place. It pretty much pulls that off, so long as you don't mind installing software on your computer, rather than having a client on a web page - so old school.
So, why would you install something on your computer, when there are no-install options with Google and Amazon's offering? Speed. Demon speed.
The Spotify client isn't just another media manager and player; it looks a lot like iTunes or DoubleTwist, but it's the fastest thing around. I've got 12,000+ tracks in my library (local files), and searching through them is literally instantaneous. Even better? The delay between when you press "Play" and when your music starts is absolutely zero, essentially removing the buffering effect of cloud-based playback...but wait...Spotify's streaming IS cloud-based playback. So what gives? Well, besides simple speculation, I just don't know how they do it. Spotify's performance is so amazing, it's literally a feature all its own. For example, my girlfriend has a computer at work that's old, and generally slow at all things multimedia on the web. She hasn't been able to use any of the cloud-based services such as Google Music and Amazon Cloud Player because it locks up her browser. Spotify, however, still streams instantly for her, even on her older computer, and the experience has her spouting so many positive things about the service that I just had to jump in and give it a go myself.
If you don't have a Spotify account yet, you can request an invite to the free service (I used the free service and it's features alone for this post), or as mentioned before, if you want to pay for the premium service now, they'll let you in right away. I got an invite in just three days, so the wait for the free service doesnt' appear to be very long.