In End User: The Three iPhone Applications That Are Keeping Me From Buying a Palm Pre, Aric Annear says this of IMEEM mobile compared with Pandora:

Both the iPhone and the Pre have Pandora, which is a great application, but the recently-released imeem app takes it a step further with the ability to upload your entire music collection to imeem and stream it to the app, so you don’t have to take up storage space on your iPhone. It also has other features from the imeem.com site, such as Spotlight artists, recommended music, and featured stations. And if you find new music you can’t live without, you can click in the app to go straight to that artist’s iTunes page. Pandora’s fantastic, but the ability to jump within one app between new music and my own uploaded favorite tunes makes this one a winner for me.

Imeem vs Pandora for the iPhone