I was pretty impressed with the ease-of-use in doubleTwist. Upon connecting my Android 2.2 phone to both my Windows and Mac devices, and setting the phone to "Mount as disk drive," I started doubleTwist and the syncing began. One thing to note: which I discovered a little late: be sure you select the appropriate storage space into which to sync. I selected the smaller SD card without paying attention and ended up filling the little thing up quickly.
A quick re-sync with the main storage on board the phone and deleting the files out on the SD card took care of this problem, but it did eat up some extra time.
doubleTwist tries very hard to be an iTunes replacement, and for the most part it seems to work. I had no issues with music purchased on iTunes playing on the Android doubleTwist client, and these were most definitely DRMed m4a files I keep around for just such tests.
The desktop app itself is very iTunes-like with a sidebar for navigation that includes (besides links to the device itself) a playlist creation tool -- even a link to the primary Android Market.
For purchasing music, there's an in-app link to the Amazon MP3 music store, which provides a very good selection of DRM-free music for purchase and consumption.
There are limitations, though. For example, you're not going to get videos on your Android through the Amazon store or anything you might have in your iTunes collection. Any videos you pull in will have to come from your on-board Android camera or be DRM-free within iTunes.
Also, and this is more of a nuisance than one would expect, there's really no Album view within doubleTwist's desktop client. You can sort by album title, of course, and you can create playlists to your heart's content, but it would be great to just tap an album cover and have that album kick off, as you can on the device.
Another great music synchronization app is TuneSync. TuneSync is a freemium app for Windows and Mac users that also taps into iTunes and syncs content over to your Android device. The difference between it and doubleTwist is transparency. Once installed as a server on your Windows/OS X PC and a client on your Android phone, this application syncs music files without any fanfare or user intervention.
Music files and playlists, for instance, are dropped neatly into your Android device as they're added to your iTunes library, where they can be played by the native Android music player.