I've had only so-so results using iTMW with my SE w800i. iTMW (Itunes My WalkMan) is an Applescript that installs a menu within your iTunes program. you create a special playlist and then run the script and your SE walkman phone is synced to that playlist. Works okay but not great for me. I found bettter results using a shareware app called Dreamsicle that syncs both iTunes and iPhoto content with SE phones.
As much as I love the w800i, I also found that Nokia S60 phones and Samsung phones were easier to use in terms of transferring music and photos back and forth with my Mac. Straight drag and drop. Memory card mounts like any removable mass storage device, and even boots up iPhoto automatically (for better or for worse). Supposedly the Cingular-branded w810i will support Mac drag and drop, but I can't confirm that.
iSync, as mentioned before, supports calendar/contact syncing with select phones only. Not music. And I could not get it to work with my w800i at all, even though it's supposed to be supported. Definitely some "Your Mileage May Vary" when it comes to OS X and mobile phone syncing.
Also, FWIW, NOT ALL usb mass storage devices play nice with Macs like they do with WinXP. I have an SE m600i that mounts and supports drag and drop of music and photos on my wife's WinXP laptop. Can't even get it to mount on my Mac. Too bad, too - it's a great phone. Maybe a firmware update will fix that.
That all being said, the SE phones are, IMHO, the best when it comes to music management on a mobile phone. Nokias are quite good, too. Samsung's sound great as well, but the UI is not as nice.
Then there's the mythical Apple iPhone which may or may not drop January 8, 2007 at MacWorld
