I am a mac user as well. The bottom line is you won't get the same level of syncing support as you did with an iPhone and iTunes - that's what makes the iPhone such a marketable object.
With that said, I jumped ship due to AT&Ts service and Apple's lockdown of the OS forcing me to jailbreak to get the functionality I wanted.
I have about 12 gigs worth of music - here is how I copied it over.
I have tried DoubleTwist but frankly think that the interface is lipstick on a pig - it just doesn't work that well. All of my music is in my "Music" folder on my HD. If you follow the steps from the people above - notably, you need to Mount the SD Card by selecting mount from the pull down menu on the top of the phone. You should see the SD card mount on your Mac. From there, simply drag and drop your entire music directory onto the SD card - that's it! When the copying is done, eject the disk and you are done.
The first time you open your music player it will search your SD card and index all of your music - regardless of what directory it is in or what it is called. After a few minutes (depending on the size of your folder) you should have your music indexed and you are good to go.
It's up to you whether or not you want to stay with the Droid - there are trade offs with regards to music synchronization with Macs - but those trade offs are worth it in my opinion.
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The first time you index, you won't be able to play any music until the phone builds the thumbnails for all of your cover art - it may give you an error such as "Can't recognize the file" or something like that - just wait and it will finish building the thumbnails and it will then play the music.