Slacker 2.0 allows you to cache your station to your iPhone, this way you can play your stations from the device memory, no Wifi or 3G necessary and this saves battery life. Slacker has about 4 times the library of Pandora. I have a plus subscription which is $3.99 per month no ads, unlimited skips.
Pandora is great too and I use it as well. But it is a far cry from Slacker, which allows you to create stations with the specific songs and/or artist that you want. Not some system that decides for me. Even though I did the Pandora one, I am still limited in terms of the number of skips.
If you have not tried Slacker give it a spin! The caching of stations makes them light years ahead of Pandora. I also understand that Slacker will be iOS4 compatible once Apple approves the the update.
OTACORB