I enjoy integrating Pandora into my music rotation along with my iTunes library and Spotify, but doing so on the desktop either means keeping another web browser window up or downloading Pandora's desktop client for OS X. Pandoras Box does a good job in condensing the service down to a menu bar app, but there are a few caveats.
Pandoras Box is a recent arrival to the Mac App Store, but the first version turned away many potential users by requiring both a paid account for Pandora and downloading the desktop client. However, a recent update eliminates both those requirements, turning it into a more useful utility for casual and power Pandora users.
It's easy to give a thumbs up/thumbs down rating from the app, or undo it, and toggle among your pre-established stations. However, creating a new station involves using an existing official Pandora client or the website. You can tweet what you're listening to in Twitter and links are provided to iTunes and Amazon for purchases.
There is the option to match songs with those in iTunes to increase play count there. Other options include integration with Last.fm, using keyboard media keys, enabling Growl notifications and launching at login.
US$4.99 might be a little high for Pandoras Box. Similar apps run around $2.99, and I think that's a fair amount to charge for features that it has.