Sync. Got room on my ipod (40gb photo), so why not take the whole collection? Plus photos, plus the hi-res versions, plus a backup of my mail and documents.
Manual. My media library is so hopelessly huge and disorganized that I don't want to go through the trouble of dealing with that mess when I'm on the go.
A little of both. I synced my whole library first then switched it to manual since my wife and I share the iPod and that's the only way for each of us to sync our playlists using different acounts.
I have friends who brought over iPod so they could load up my music on their players, as they are trying in the meantime to build up their libraries. So I manual sync up, cause some of my friends dont like Country.