I have about 12 gigs of music on my 64 GB. I find it useful for streaming to an airport and Apple TV without needing to have a mac on all the time instead. The iPad seems to stream more reliably than the mac or iPhone for some reason, maybe the antenna is better?
Also when travelling, it's easier to just be dealing with one device on the plane...so I can read an ebook whilst listening to music from the iPad and keep the phone switched off.
I also like the iPod interface on the iPad, it's nicely laid out. Having 64gb makes all this possible though....if I had less memory I might do things differently.