Think of it like wiping OSX from your Mac and putting Linux instead. It's definitely not for everyone, but it's also far easier than the average person might think.
The main benefit from my perspective is NO MORE FUCKING ITUNES. Just plug in and copy songs as if it was a standard USB drive. BIGGEST WIN.
The flow-on benefits of this are countless, eg
no more piles of shit like Quicktime being loaded on your computer
no more needing to load ANYTHING on your computer to transfer music
no more "oh you want to add songs from a different computer? let me wipe all your songs first"
no more "oh you want to copy your songs from your ipod to your computer? I can't hear you la la la la"
no more "oh you want to listen to your music? Too bad, corrupt library lol"
The other key feature is plugin support. Most common plugin use is to play different formats. I admittedly only use MP3, but it CAN play OGG, FLAC, APE etc. The main plugin I use is to 'scrobble' my play history to last.fm. There's also plugins for advanced DSP, as well as less useful things like ebook support and being able to play a low-res port of Doom with your clickwheel... eh... anyway...
Aside from that, little things like being able to skin/theme the interface are a nice bonus as well.