Having only just replaced my iPod (80GB classic) with my phone (Galaxy S), I was hoping to receive similar functionality via Spotify. However, I find offline listening to be incredibly clumsy! I find that having one playlist full of music I wish to sync just does not work. In the mean-time, I have resorted to creating one playlist per album. This still isn't great, but is a marked improvement.
I think that there's a better solution to this problem.
I propose that browsing your offline files becomes a little more succinct, and similar to the functionality of other music players on the market. The simple ability to browse your offline artists/albums/genres would improve the usability of this app to a huge extent.