If you're thinking about Spotify, pony up and pay the $10 / month. You'll get access immediately instead of waiting in the invite queue for the free version. My (short) adventure with the free version is below.
I have a beta invite to the free version of Spotify, a music streaming service that just debuted in the US. Basically, Spotify lets you stream music from a huge library. There is also a paid version, but I wanted to give the free version a try.
I downloaded the app on my PC and allowed it to connect to iTunes. I installed it on my iPhone and I was able to see all my iTunes playlists - cool! See I can see all of the songs that I've put into playlists, I can stream them, right? Especially since I own them? Apparently not with the free version.
It turns out that the free version of Spotitfy is only useful for streaming Spotify's catalog on a PC or laptop, or downloading music that you already have onto your iPhone. The latter is is exactly what you can already do with iTunes, albeit this is a wireless solution. In other words, the free version is useless.
I upgraded to Spotify premium for $10 a month. For the price of an album, I can stream from the Spotify catalog on my iPhone, as well as replace iTunes as the syncing device for my purchased music. The best hard is that the songs queue up quickly over 3G, so much so that it feels like the songs are stored locally on the device instead of Spotify's streaming servers.
There's also the option to download songs locally if you'll be without internet for a while. This is good if you can predict a bad internet connection - but how often does that happen? My advice is to always keep a core selection of songs on your phone, and use Spotify to access songs that pop into your head during the day. That alone is worth $10 a month.
Bottom line - you should expect to pay for a great service, and Spotify is just that.