The Main Window
There's nothing special to say about Songbird's look other than it's mostly like iTunes, but with the main toolbar at the bottom. This can be switched to the top, via "View > Player Controls > On Top". The other bar at the top is the tab bar, allowing you to browse and search websites.
The various display panes (mashTape / Album Art) can easily be hidden by the docking buttons below them. Songbird is extra flexible though, letting you get add-on side panes such as a folder view or lyrics browser, as well as swapping around which side-pane module is shown in each of the panes. This flexibility and easy manner to get new modules for the panes should allow for a lot of innovation to happen outside of the main Songbird development.
The genre browser has a different kind of button to hide it which is quite cryptic and not immediately obvious. It's left of the search bar, but you can always choose the "View > Media Views > List View" menu.
mashTape can help solve a lot of your curiosity about various songs and artists, automatically pulling in (hopefully related) info from the web.
The "Photos" tab finds pictures from Flickr. It seems like something that would entertain YouTube users, rather than a feature I would actually want to make use of.
Any audio player lives and dies based on its ability to play audio. Songbird's wide support for audio files is going to please some people. Between users hurt by Apple removing FLAC from iTunes, and supporters of open formats like OGG - Songbird caters, but Songbird even plays nicely in a proprietary world; it being able to play DRM protected tracks via hooks into QuickTime and Windows Media Player, as appropriate.
In the mini player view though, I did get odd error messages saying that the song could not be played because it was encrypted, yet it was already playing fine. Clearly just a minor bug.
There is no cross-fade support yet, and I encountered a number of jittery moments where songs cut off a second or two early, or the player just stopped entirely after a song and hung there on the next song waiting for me to hit the top of the box to kick it back into playing.
I miss not having iTunes simple party shuffle mode, and when I do shuffle the play order in a playlist, the focus doesn't follow the current song as I skip
Overall though, if your library is diverse and you're more particular about the formats you store your music in, Songbird will work well for you. Songbird plays generally well with your iTunes purchases so there's no real reason to not try Songbird out for yourselves.