I just purchased iSyncr for my iTunes music collection and it's relatively easy to copy all of your music over to the phone. After you install iSyncr on your phone, you connect the phone to your computer(be sure to activate the USB connection), if you are a Windows user, you will see your phone listed as an external drive or two if you have an SD card mounted. There is an iSyncr executable in the external SD card, click on that and a window will pop up that allows you to select which iTunes category you wish to sync, in this case click on music and click on the button at the bottom of the window. iSyncr will read your iTunes music directory. When it is finished, click on the Sync button and wait while the files are read and copied. iSyncr won't copy any music with a DRM lock. When you add new music to iTunes you can go back an sync just the new files(there is a check box)
When you install iSyncr on your phone, there is tutorial that will walk you through the syncing process.